When a pawn advances to one of the four hexes at the far edges of an opponent's tridrant (a2, b1, c1 or d1), it must be exchanged for a piece of higher value, usually a queen. (You may not exchange a pawn for a king.) You may have more than one queen when you promote pawns. An upside down rook can serve as your second queen.
Bottom: The promoted pawn is exchanged for a queen.