In chess, "relative value" refers to the estimated value of the various types of chess pieces in relation to each other. These relationships are typically measured in pawns.
In the most typical schema, the queen is said to have a relative value of approximately 9 pawns, the rook is worth approximately 5 pawns, and the knight and the bishop are worth approximately 3 pawns. A pawn is of course worth exactly 1 pawn.