[Note: The frame data found in this entry was taken from NKI's translation of T.Akiba's page.]
When your opponent blocks an attack, they become stuck in the blocking animation for a short period of time. During this time, they are unable to move, attack, jump, or do anything else. This is called block-stun.
When you hit an opponent in the face with a move, pretty much the same thing happens. While the opponent is recovering from being hit, they cannot move, attack, jump, or even block! This is called hit-stun.
Your opponent rarely recovers from being hit at exactly the same time that you finish your attack. One character usually recovers first. If you are the one to recover before your opponent does, we say that you have frame advantage. When the opponent is the one to recover first, however, we say that you have frame disadvantage.
In the diagram above, you can see that Ryu recovers 6 frames before Dhalsim does. His crouching strong is finished once the recovery frames are over, but Dhalsim is still locked in hit-stun at that point. Basically, this means that Ryu has frame advantage in this situation. He can start acting 6 frames before Dhalsim can.
To communicate this information quickly, people usually express frame advantage as a number, using "+" or "-" to indicate advantage or disadvantage. For example, we might say that Ryu's crouching strong is "+6 on hit," meaning that Ryu recovers 6 frames before his opponent does.
But why that "on hit" part? The numbers are usually different when a move is blocked vs. when it hits, which is why you'll see expressions like "-5 on block," "+2 on hit", and "+7 on crouching hit." It's not necessary to memorize all the numbers, but understanding the concept of frame advantage and disadvantage is important.
For example, if you do a move that is -5 on block (and your opponent blocks it), they will recover 5 frames before you do. That gives them 5 frames to attack, where you can't do anything to respond! Worse, If they have any moves with less than 5 frames of startup, they can use them at this point to hit you for free. Remember, you're stuck recovering for 5 frames, so if their move will hit before that, you won't be able to block it..
