If a pawn should by chance manage to make it to the eighth rank (opponents back row rank) the reward is an exchange for any back row piece except a King. We call this promoting the pawn.
Usually (almost always) a Queen is requested. Except in rare circumstances, it makes little sense to ask for a lesser piece. If a Rook, Knight or Bishop is requested rather than a queen, it is referred to as under-promotion.
Many endgames involve promotion races or a struggle with one side trying to promote and the other trying to prevent the promotion. The outcome of these races or struggles for promotion will decide the result of the game. So pawn promotion is a pivotal part of chess.