Promotion is what happens when a pawn reaches the other side of the board (that is, the 8th rank) and cannot move further. Then it becomes any other piece on the board, except the king. Players usually promote their pawns to a queen (called "queening") because it is the most powerful piece on the board. There are cases where a player might want to promote a pawn to a knight, rook, or bishop instead. This is called an "underpromotion", but they almost never happen, as almost all promotions are made to a queen.