We know that pawns can only move forward, not sideways or backwards. If not blocked by a pawn or piece, it will eventually reach the end of the board while moving forward. Now the question is, what happens when a pawn reaches the end of the board [for white pawn 8th rank or black pawn 1st rank]? Answer: When a pawn reaches the end, it can no longer be a pawn, the pawn is promoted.
If someone’s pawn reaches the end of the board by moving forward, it is called pawn promotion. And it can become any other piece except a pawn or king. The player can take a piece as per his wish. Players usually take a Queen instead of a Pawn, because the Queen are the most powerful piece in the chessboard.
The white pawn can promote a queen by moving the pawn after two moves.
But sometimes it is seen to take another piece instead of the queen. After the promotion of a pawn, if he is transferred to any other piece instead of the queen, it is called under promotion. Underpromotion is not commonly seen but this is useful in some special cases.
Here white to move. If the white will promote a Queen then the black king will be stalemate and the game will be drawn. So here white will promote Rook for winning the game.
