When a pawn advances to one of the five hexes at the far edges of an opponent's tridrant (a3, a2, a1, b1 or c1), 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.

drawing of the 3P3 board with pawn promotion shown
Top: Red pawn moves to a3B (a-file, 3rd rank, Black tridrant).
Bottom: The promoted pawn is exchanged for a queen.