The value a piece is worth at a particular time. Examples at the beginning of the game:
Pawn = 1 point, Knight and Bishop = 3 points, Rook = 5 points, Queen = 9 points, King = priceless
A value may also be given to pieces in conjunction to others. One example, is that in the endgame, a pair of Bishops may be worth more points than a Knight and Rook.