Here's the tl;dr: "absolutely everything".
Every game has its own culture, and a part of that culture is being able to understand what's going to happen based on the current conditions. It's not something you can read up on and understand. It's a sense you get from thoroughly understanding your environment, the mindset of the players around you, and the mechanics, capabilities, and rules of logic of the various constructs within the game.
The tricky part of all this is that most of us accomplish all this naturally over time. We don't even think about it. One day, it just "clicks" and we understand the flow of the game enough that we can anticipate what's going to happen. "That's bait; I'm going to avoid that fight," or "This is a hot-drop situation," are overall senses about which we can't point to a single piece of data. They're deductive conclusions that we arrive at after assimilating and processing a wide range of information.
One of the interesting parts of this, for me, is that this process is identical within Eve as in the real world. When driving, a veteran driver is better able to predict and exploit the flow of traffic to change lanes, and we're always better at making good time during our rush hour commute than other roads, since we know the "flow" - where people are always log-jammed when coming onto the main road, where the left lane moves faster, and where you may need to push the person in front of you a little bit to make that long light.
In Eve, these kinds of insights are difficult for a newbie to attain. They require you to be familiar with a wide range of flying situations, the overarching fitting theories prevalent at the time, and the way various ships actually work in combat. That means getting out there and trying new things. The old saying is, "To learn PvP, fit up 100 frigates and lose all of them."
But why does experience matter so much?
Let's assume you've read extensively about the various ship classes, and have memorized each of their bonuses and roles. Let's assume you understand about kiting, fitting tactics (ie. why you don't active and passive tank a ship at the same time), and know all about all of the commands and capabilities of ships in general in combat. You've learned all about the different weapon systems, about transversal and falloff, and know what all of the modules do.
By the way... no player can claim to know all of that. I still need to look up the bonuses of some ships before I fight them. Our age of constant ship rebalances and tiericide means there's quite literally new information and bonuses with every six-week patch. But let's assume you know all of that.
What you won't know is how well a command destroyer can slip under a cruiser's guns. Can your cruiser track a frigate orbiting tightly when you're webbing him? What about two webs? How long can your tank last against blasters, against pulse lasers, against heavy missiles? Can a 100mn Phantasm turn fast enough to keep pace with a ship under MWD? How fast does that Phantasm accelerate to max speed? Can your Crow avoid an instalocking gatecamp? What about if you're in AUTZ instead of EUTZ (and because you're located much further from the London servers have a much longer ping)?
These are the kinds of things experience will teach you that you can't read in a manual or wiki, and they're different for each ship. Only by flying the ship in space can you really understand its capabilities. In a game that relies - in some cases - on split seconds to determine if you will warp this server tick or your ship will be pointed on the next, you need to get a feel for how your ship flies.
To use an old example, many people uses to ague that you should start with a Stabber, move to a Stabber Fleet Issue, then a Vagabond, then a Cynabal. That logic was a bit foolish, though, as a Rupture actually was a fairer approximation of an "entry level Cynabal" than a Stabber was. And a Vagabond was a bit clunkier and less nimble than a Cynabal. In many ways, an Omen Navy Issue or Vexor Navy Issue were better approximations to a Cynabal despite different weapon systems. But the only way you could know that would be by flying the ships and getting the feel for them.
And that's only focusing on your ship. By flying a lot of different ships, you can get a sense of how your opponent may fly them. If you've tried fitting up a Caldari ship, you've no doubt noticed the glaring EM hole in the shields. Convention has it that you need to fill that EM hole, which requires at least one slot. While counter-fitting - fitting missiles on an Omen, for instance, or lasers on a Rupture - is certainly a thing, those fits tend to be made for specific scenarios that you're unlikely to face during a typical roam. As you fly ships, you get a sense of what works and what doesn't work, which allows you to make some assumptions about your opponents' fits.
But fitting tactics and the handling of ships aren't the only area where experience is invaluable; it also helps with the psychology of players.
If you play Eve long enough, you start to realize the tell-tale signs of the motivations and behavior of your opponents. You may have heard some of your corpmates talk about ships that are clearly bait - a battlecruiser sitting on a gate for a long period of time, or a Myrmidon of any variety, or a pilot ratting in a busy system but not actually producing any wrecks. By experiencing a variety of situations, you can begin to understand the mindset of those players. If a pilot isn't producing wrecks while "ratting", they're obviously not fit to to damage to rats sitting at range. A Myrmidon is known for being a dual- or triple-repping monster. A pilot sitting on a gate likely has backup nearby and is deliberately attempting to attract attention. A Brutix Navy Issue is almost certainly hull-tanked and can absorb about 200k of damage.
Often, I hear players complain about being repeatedly killed. Reddit had one yesterday where a pilot was considering unsubbing because he lost two Tengus in 24 hours. The fault, my friend, isn't with the game. It's a lack of experience that led to you making mistakes that cost you your ship. The remedy isn't quitting; it's in understanding what you did wrong and what the people who killed you did right. Maybe you were predictable. Maybe you didn't fit a nullification subsystem as you traveled. Maybe you weren't paying attention to dscan.
Each of those lessons builds a sense of your understanding of the artificial world of Eve, the mentality of its denizens, and the behavior and decision-making that the pilots you meet are likely to believe will lead them to success. Like the moment you learn to read, after gathering enough of that experience, it will one day all click into place, and you'll understand what's going to happen before it does. Your subconscious mind will recognize the patterns you've seen before and correctly put them together to anticipate the situation. Knowing where various groups stage out of and whether they tend to hot-drop can tell you whether that ship you're about to engage is really alone or not.
Three days ago, I was bringing in a triple-rep Stork through six jumps of null-sec when I landed in a bubble 30 km away from my out gate. Sitting 80 km away from me was a Nightmare, a ship I've never before seen in a PvP environment, let alone solo. He fired at me and took my shields down to 75% as he burned towards me.
At this point, I had a choice; should I engage him and attempt to hug a tight orbit, or should I continue burning to the gate and jump through? Typically when I see a battleship and I'm in a destroyer or frigate, I'd feel pretty confident that I should engage close and wear him down. A Nightmare has a 75m3 drone bay, but I felt confident my three reps could hold up to three flights of light drones as my missiles destroyed them.
But I paused. The Nightmare knew what ship I was in, and yet had attacked me nonetheless. He obviously felt confident that he could kill me, despite the apparent vulnerability. Did he have friends nearby? There were others in local; perhaps some of them were cloaked up nearby waiting for me to aggress. Perhaps he was fit with a scram and two webs, and felt confident his 100mn AB would allow him to dictate range enough that his guns could hit my ship as it barely inched through space.
In the end, my experience told me not to take the fight. After I jumped the gate and continued on my way, I asked a few alliance mates how well a Nightmare's guns could track (pretty well) and reviewed the slot layout, which left me pretty confident I made the right call. Had I not seen enough situations in both killing and being killed by small ships, I'd probably have engaged and lost a Stork.
There's a tendency in our world to desire purity, perfection, and the beauty of the untouched and unblemished. We forget that those blemishes and scars represent challenges survived and lessons learned. The undamaged mind is a mind that has learned, experienced, and overcome nothing. "Childlike" shouldn't be a state to be admired; it's a state to be overcome.
That's why experience is so critical in Eve, and why players without it find PvP so frustrating. So much of Eve is about choosing your fights carefully. It's not really that hard to tell if a pilot is linked or not by his speed, how far he can point you, and how many people are in local. If he doesn't follow you into the next system, chances are he's linked. People talk about skillpoints as the key advantage veterans have over new players, but in reality, their advantage comes from understanding the "flow" of the game.
And, do you know what? Just as you can understand that flow, you can also lose it if you don't practice regularly. A pilot who doesn't log in for a few patches is going to lose the sense of what their ships can do, and within that lies your advantage. A pilot engaging in PvP regularly is always going to have an advantage over one who doesn't, regardless of sp counts. A smart player who flies a lot will have that understanding of the flow of the game, tempo, and situational familiarity that will overcome any advantage, since it will allow that pilot to decide when to engage and when to pass.
A linked pilot can't kill someone who refuses to engage. A fleet can't hot-drop who recognizes a ship as bait. A gimmick-fit Nightmare can't kill a Stork that won't aggress and instead chooses to jump out.
Don't be discouraged by losses in PvP. If you're smart and you're learning on T1 frigs, the costs are minimal. But - more basically - don't view your wrecks as losing ships, but rather buying memories. Buying experiences. Eventually, you'll tap into the flow of combat and begin to take control of your losses and kills. That first solo kill against a competent opponent brings with it the thrill of combat matched with awareness of your accomplishment.
And that's an expreience unlike anything else.