A metric that looks at how teams perform with a certain player on the court, how they perform with a certain player off the court, and calculates the overall impact that player has on team success.
This is also sometimes referred to as the on court/off court number.
Plus/minus began as a hockey metric, and has been kept as an official NHL statistic since 1968. Around 2003, Roland Beech of 82games.com brought this concept to NBA basketball...and a few years after that, the league made +/- a part of it's official boxscore statistics.
Interestingly, this metric is much more meaningful in basketball than it is in hockey, even though it took much longer for the NBA to begin using it. The more scoring there is in a game, the more useful this statistic becomes. Plus/minus is a useful way to determine a player's value to his team, because it encompasses everything a player does on the basketball floor, even things the NBA doesn't keep official statistics on.
An example of Plus/minus is as follows: let's say a player's team was 7.5 points better than their opponents when he was in the game (+7.5). When our player was not in the game, however, the team was 3.3 points worse than their opponents (-3.3). To find this player's +/- rating, you simply take the difference of the two numbers, which yields a rating of +10.8.