Player efficiency rating (PER) is a per-minute, all-in-one box-score rating created by John Hollinger: it adds up a player's positive contributions, subtracts the negative ones (missed shots, turnovers, fouls) and scales the result so the league average is 15.
HoopSim shows PER in the Advanced view of the player stats pages, computed from the box score with Hollinger's coefficients and scaled to a league average of 15, without the original's pace adjustment. Next to it HoopSim also shows box plus-minus (BPM), value over replacement (VORP) and win shares (WS), so PER is one all-in-one metric among several rather than the game's own. None of them is a rating: a player's true ratings stay hidden behind your scout's read, and stats are the one thing every GM sees exactly.
Rulebook: The box score and play-by-play. Related: plus-minus, usage rate, OVR.