You can think of it as a way of measuring the intensity of a movement/activity by the way it feels that day. This is like using percentages as a guide but it accommodates for daily fluctuations in energy, recovery, and performance level that percentages don’t. Ever feel like a certain weight is way heavier one day compared to the last time you lifted it?
As an example, if the recommended intensity is an RPE of 8 for a set of 5 deadlifts, you would use a weight that allows you to do 5 reps in the set, but leaves two reps in reserve (RIR), meaning you could have done a set of 7 at that weight before failure. A set of 5 at an RPE of 10 means you can do 5 reps at that weight and no more. Below is a helpful chart to understand RPE's relation to RIR.
When in doubt, ask one of your coaches for guidance and they will help you out!🤝