Defensive Plus-Minus is a metric that measures the difference per 100 possessions in points allowed with a player on the court versus off the court. The accuracy of this measure varies depending upon how often the player is on the floor and whom the player share it with. In most cases, it provides a good indication of a player’s overall defensive value to his team.
If the player’s team allows 110 points (per 100 possessions) while the player is on the court and 105 points (per 100 possessions) while he is off the court, his net defensive plus-minus is +5 (110-105=+5). It’s better to have a negative defensive plus-minus value.