RPE stands for Rating of Perceived Exertion. In other words how subjectively hard an exercise or workout is relative to the person doing it.
Trainers/Coaches use RPE with clients to help with programming or making exercises easier or harder.
0 - 3) Nothing at all, light/easy.
4) Recovery, not hard, high rep sets. *RPE below four isn't important. (1)
5 - 6) Moderate intensity. Most warm-up weights.
7) Hard, but maintainable, 70% intensity/capacity.
8) Heavy/Hard intensity, but 2–4 reps left in the tank, 80%.
9) harder, but still one rep, or a little gas left in the tank.
10) Very hard, maximum intensity nothing left. 100% effort.
A simple scale of 1-10 is used, with 1 being easy, think sitting down relaxing. Whereas a 10 means you’re going to pass out, vomit crap your pants and/or both. A level of activity/exercise that is extremely intense and that you can’t keep up. Think maximal effort sprinting/exercise or weight lifting.
In order to elicit specific adaptations to be made, like strength or positive body composition changes (like more muscle) you generally want things to be on the higher end of the RPE scale for some, but not all of your workout.
Being that the basis for the RPE scale is perception, this generally means that the person should have a decent level of self-awareness related to exercise, rather than someone totally new to it. Though I still love to use it with beginners and or people who tend to surrender too early in a set. Why?...
Because good programming has slow, adaptable, forced progression in it. So even if a person cries out that an exercise was super hard, 10/10 difficulty, we slowly and safely progress them 1 rep or weight over time to the point where it is no longer difficult. ⠀
*SOURCE: (1) Tuchscherer, Michael "The Reactive Training Manual: Developing Your Own Custom Training Program for Powerlifting." c. 2009