Frame data
How many frames a move takes to start up, hit and recover from — fighting games' exact currency for what is safe and what combos.
Every move in a fighting game breaks into startup (frames before it can hit), active (frames the hitbox is live) and recovery (frames before the character can act again). The difference in recovery between two players after a block — frame advantage — decides who gets to act first next. It is all counted in frames because the game runs on a fixed timestep of 60 frames per second.
At a high level, frame data is not trivia but the language itself: a move that is "−5 on block" is unsafe, one that is "+2" keeps the initiative. Players memorize the tables the way a chess player knows openings, and hitstop, i-frames and input buffering are counted in the same unit.