RPE means Rate of Perceived Exertion: a 1–10 score for how hard a set felt. In modern strength training it is usually tied to reps in reserve (RIR)—how many good reps you could still do. RPE 10 means no reps left; RPE 9 means about one left; RPE 8 means about two left.
Coaches use RPE to autoregulate load instead of only prescribing a fixed percentage of a one-rep max. For example, “5 reps at RPE 8” means stop with roughly two reps in the tank, then adjust the weight on the next set if the set felt easier or harder than planned. RPE began as Borg’s aerobic effort scale and was adapted for lifting (notably by Mike Tuchscherer) so day-to-day fatigue is reflected in the working weight.
Source: A Guide To Using RPE and RIR In Your Training — Ripped Body.