From IFWiki

Backstory is the history that provides context for the narrative of the game, but is not itself part of the said narrative. This might include information such as the history of the world in which the game takes place, the past of the player-character, the mechanics of a magical system, etc. Often the backstory is developed further as a result of the outcome of the game. In some games, reconstructing the backstory -- finding out what happened in the game world before the game began -- is one of the game's goals. See for example Babel or Anchorhead.