Overall sum of changes in the difference of the score while a player was in the game.
Example if a player played 10 minutes in the 2nd quarter and the team was behind by 4 point then was up by 4 when he was subbed out his +/- would be +8. And then he played the last 5 minutes in the 4th quarter and the team was up by 2 but lost the game by 1 (phantom 1 second drive buzzer beater FTW!!!) His +/- would be -3 for the amount of time but +5 for the whole game.