When a pawn advances to one of the four hexes at the far edge of the chess board, the player must exchange it for a piece of higher value, usually a queen. You may have more than one queen when you promote pawns. An upside down rook can serve as a second queen.

drawing of the 4-pawn board with pawn promotion shown

White pawns promote at: a9, b9, c9 and d8.
Black pawns promote at: a2, b1, c1 and d1.