The plus-minus statistic originated as a measure of a player's impact on team performance in hockey, but has since become a popular metric in basketball. It measures the difference between a team's total scoring and the opponent's total scoring when a particular player is on the ice / court.
In theory, the plus-minus statistic incorporates intangibles that aren't accounted for otherwise. For example, in basketball, things like deflected passes aren't always counted as steals, but can make a big difference in slowing momentum. It also captures great defensive efforts and other hidden contributions like setting effective screens. The downside is that it's highly volatile with small samples and is significantly impacted by other players. To mitigate this, many are looking at real plus-minus statistics instead.
Lastly, in addition to evaluating individual players, many teams use plus-minus statistics to identify productive lineups.