Basically, it means how delayed or how preemptive the next action of both players is. If you use a move that is +6 on block, you have a six frame head start and if you use a fast attack can snuff out most attacks your foe can try to use(this is called a frame trap). If your foe is in a similar situation or you just had a move that is highly negative(let's say -16) on block blocked, chances are your foe will be able to hit you with a fast move before you can block or attack again unless they react too slowly or something else happens like they JG.
Starting off with frame data, you'll want to focus on only the largest differences. This will give you an edge over most online players, where smaller frame advantages and disadvantages are negligible the overwhelmingly vast majority of the time.
As you delve deeper into it, you want to compare frame data of different charaters so that you can make use of even the smallest positive or negative of all attacks. This requires a lot conscious effort(if you react even one frame late at times you may be severely punished for it) and a crapload of memorization.
Quick! You're playing Astaroth and just had your 44[A] guard crush blocked. Which attacks can you safely perform without Pyrrha stabbing you?!? You need to worry about both how fast the startup and how long the recovery time of your next attack is to know which attacks you can use to exploit your frame advantage.
But now we're discussing things nearing tournament-caliber play. You'll need to learn frame data for pretty much every character in the game to make use of those small +3 and -4 and such frame advantages, as well as pushback on attacks and how some attacks are slightly slower at tip range, how they move you, and so on...As hard as it sounds...it is! All of this more technical, in-depth data is also less useful online where folks like myself largely ignore those smaller frame advantages and even the slightest input lag can ruin your perfectly executed punishes.
If you don't intend to play against tournament-quality players, going deep into frame data won't be very useful for you. If you *DO* intend to enter tourneys though, it is essential knowledge.
Oh, and being at negative frames doesn't mean you shouldn't ever attack. If you're not severely negative(let's say anything worse than -11), many characters have moves that are great for the situation(Mitsurugi 4B, Algol 623B, et cetera), and there are always GI and JG to fall back on.
