An example of Underpromoting a Pawn (Bishop promotion)
In chess, promotion is the replacement of a pawn to a queen, bishop, knight, or rook of the same color after that pawn has reached its last rank (8th rank for White and 1st rank for Black). The player can choose from any of the above pieces, but may not choose another pawn or a king. Usually, the queen is chosen (queening), but in some positions, another piece is required to avoid losing or stalemate (underpromotion). Most players underpromote to a knight, as its movement cannot be replicated by the queen (or any other piece, for that matter). Some chess sets come with an extra queen for each color. If not, usually players will take a rook and flip it upside down, representing a queen, if they promote to a second queen in the board.
An example of Promoting a pawn
Passed pawns close to promotion pose a threat for the opponent in the endgame, where there are less pieces. Many of the opponent's pieces will attempt to protect the promotion square or threaten the pawn, and the player with the pawn can use this to their advantage. It could also be beneficial to bring several pawns near promotion, if one pawn is captured, the other pawn can be promoted.
The rook cannot stop both of the pawns from promoting
