A variable in RPG Maker is an internal number used by an RPG Maker game. Variable values can be changed by events using event commands, and can also used for calculations and determining if parts of an event should be executed.
A common use for variables is to introduce a secondary currency into a game. Variables can also be used to track the amount of times an action has been done, progress with a quest or storyline event, or a number that the player has inputted in the game.
Variables were introduced in RPG Maker 2000. In RPG Maker MV, a single game can have 5,000 variables.