Because of the rules of the game, some chess pieces are more valuable than others. The relative strength of different pieces is described by the concept of material. Material advantage comes into play when all other conditions are equal. In a game of two expert players, even an advantage of one pawn can be very significant.
Chess theorists quantify the power of various pieces by assigning them different numerical values. The pawn has a value of one. A knight and a bishop each have a value of three. A rook is worth 5 points and a queen is worth 9.
Pieces can have more power or less power depending on their position on the board. For example, having a pawn deep in the enemy area may be worth much more than having a knight blocked in a corner. A bishop constrained by its own pawns is much less valuable than a knight that can roam for free. Obviously, all of these values are very relative and change multiple times over the course of the game.