Plus-minus is a simple, brutal stat: look at the score when the player enters the court and when they leave, then compute the difference. If their team wins +10 during their minutes, their +/- is +10.
Over a single game, it's a very noisy stat: one lucky run can inflate or tank the number. But aggregated over a season or career, +/- becomes an extremely powerful impact indicator.
More sophisticated variants exist: Net Rating (points per 100 possessions), BPM (Box Plus-Minus), or RAPM (Regularized Adjusted Plus-Minus). They try to strip out noise and isolate the player's own contribution.
A significantly positive +/- over a long stretch points to a player who helps their team win, even if their individual box score isn't flashy (think Draymond Green).
Real example
A player enters with the score at 40-38 and leaves when their team leads 62-54. Their +/- on that stint is (62 - 54) - (40 - 38) = +6.
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