The frame is the standard unit of time in a fighting game. As the name implies, it is a single 'snapshot' of animation. Most modern games run at 60 frames per second; thus, a frame is usually 1/60 second long.
Frame advantage Edit
Frame (dis)advantage is the time difference of stun between two characters after a move is executed. Usually this number is the difference between recovery and blockstun. A positive number indicates a frame advantage for the attacker; a negative number indicates a frame disadvantage.
For example, if Player 1 makes an attack with 10f of recovery and his opponent stays in blockstun for 12f, we say that Player 1's frame advantage is 2f. Conversely, if he makes an attack with 12f of recovery and the opponent stays in blockstun for 10f, Player 1's frame advantage is -2f.
Frame advantage is very important when weighing in the 'safety' of a move. Moves with good frame advantage are generally referred to as pokes.