To elaborate further, frame advantage and disadvantage refers to the "recovery" time after you execute an attack.
Fighting games 101:
Whenever a character attacks in a fighting game, it doesn't just hit the instant you press a button. Every attack has an animation. Virtually all fighting games are displayed at 60 frames per second of animation, so when we refer to frames, it refers to the length animations that happen when you press buttons.
Frames to hit: This is the number of frames that must occur in an attack before it can actually connect with someone (to hit, for short).
Collision frames: Once the "to hit" frames have occured, then come the collision frames. This is simply a term of convenience I use to refer to the the length of time in which a hit-box on a move is active. They vary greatly depending on the attack.
Recovery frames: Once the collision window has passed (or the attack is blocked), this the length of time that must pass before you are allowed to do anything. During recovery you are generally totally helpless to whatever your opponent can do, so the shorter this is, the better the move.
Hit stun and Block stun: Terms you will see a lot in Street Fighter, but here the concept is the same. Basically whenever an attack connects, there is a certain length of time your character reels from the blow (even on block). Think of them exactly like recovery frames. The length of these also vary greatly depending on the attack that hit or was blocked.
Frame advantage and disadvantage is a combination of the recovery frames on your attack and the hit/block stun that it creates. This is one of the ways you determine what the good attacks are. Generally block stun is the important one, as it is extremely rare (and almost always unintentional) that an attack will actually leave you at a disadvantage on hit (though it does happen, see Raphael's WS B in Soulcalibur IV for details.)
Safe: A catch-all term for anything you do that the opponent can't effectively punish you for. At the most basic, any time you are left at 0 on block, it means that you and the opponent recover at the same time.
Frame Advantage: Any time you recover before an opponent. Happens all the time on attack. Usually attacks that have a lot of to hit frames and thus are easily interrupted will have advantage on block.
Frame Disadvantage: Any time you recover after your opponent. How much this actually matters depends entirely on what your opponent can do with the time he has available.
Frame data is entirely character dependent, and specific to each matchup. What is considered safe is all about what your opponent can punish with and how.
How this helps people in fights is pretty much the same as knowing an opponent's move list. If you know what is safe, what can be punished, how it can be punished, and what you are risking when you attack, you stand a much better chance of winning than the person who doesn't know these things, and just throws out attacks randomly until something connects.
If you know that doing Flash's f2,1,3 leaves you at a +11 on block, and your opponent doesn't, they will try to attack you after blocking it. Since you recover far earlier than they do, you are guaranteed to interrupt anything they do.
