Plus-minus measures the point differential for the team when the player is on the court, which is the difference in the points scored for and the points scored against. This is largely a team statistic, but when looked at individually can show how the team performs when a particular player is on the court.
More about Player Plus-minus (+/-) Stats
The one drawback of plus/minus statistics is that it's influenced by the other players on the court as well. A mid-tier player who is typically on the court with a superstar will see an inflated plus/minus ratio. On the other hand, on a multiple game-by-game basis this statistic can be useful in determining whether or not certain players have success when they are on the court together.