Plus-minus (+/-) is the team's point margin while a player was on the floor: if the team outscored its opponent by 7 during their minutes, they are +7. It credits players for what happened around them, good and bad, without caring who scored.
HoopSim records plus-minus for every player in every game from the engine's play-by-play, since it knows who was on the floor for each basket, and totals it on the player stats pages. Raw plus-minus is noisy over a few games and depends on the teammates a player shares the floor with, which is why HoopSim also shows box plus-minus (BPM), an estimate of a player's own contribution per 100 possessions. Your closing five's plus-minus in crunch time is the quickest check on whether the right players are closing.
Rulebook: The box score and play-by-play. Related: player efficiency rating, net rating.