This is the value of pieces, only in its general senses. Actual value of pieces changes between stage of the games. It also changes based on how effective each pieces are in the actual game (ie., Bishop is more effective at 5e position rather than in 1a positioin) This chart can be used at the beginning to evaluate if those exchange you are about to make do make sense or not, and also at the middle of the game to see if you are winning or losing.