What "underpromotion" means in chess

Underpromotion is a situation where a player chooses to promote their pawn to a piece other than a queen, such as a knight, bishop, or rook. This is usually done to avoid stalemate or to create a more complex position on the board. Underpromotion is a legal move in chess, but it is not commonly used as promoting to a queen is usually the most advantageous move.