Rate of perceived exertion. How many reps left in the tank? If you could do 2 more reps, subtract 2 from 10, equals RPE 8. If I tell someone that was an RPE 6, they would know I had 4 more reps in the tank. It’s a way to communicate the level of approach to failure, RPE 10 meaning I could not have lifted one more even if my life depended on it.