RPE stands for "rate of perceived exertion". The rate of perceived exertion (RPE) scale is a technique used to express how hard you feel you are working during exercise. You can also think about RPE is "effort".
When using the 1-10 RPE scale, there is a correlation between your rating level and the number of reps in reserve (RIR) or the number of remaining repetitions that you could do with good form before failing.
For example, when we ask you do to an exercise at an RPE of 7/10, we want you to perform the given repetitions at a weight that would yield a 7 out of 10 effort. In this example, this means that you would have 3 RIR; meaning that after you performed the prescribed amount of weight you could perform an additional 3 repetitions if you had to.