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.

drawing of the 4P3 board with pawn promotion shown
Top: White pawn moves to a2R (a-file, 2nd rank, Red's tridrant).
Bottom: The promoted pawn is exchanged for a queen.