Game Theory is a study of choose, where game players are only allowed to choose / change their own strategy in the game.
For example, Player A and Player B would play a game, and the result of winning and losing would fully depended on the choose of their own strategy.
For example, In prisoner's dilemma, Prisoner A and Prisoner B are only allow to choose their strategy which is "Silent" (means to denial) or "Betray" (Means to give information to Police). The decision on Silent and Betray is the only option for the 2 prisoners. Each choose produce a different outcome.