RPE is a scale that ranges from 0 - 10. 0 is the lowest effort, and 10 is maximal effort. Basically, the RPE scale represents an individual's feeling of effort - if they were to tell someone who know nothing about the sport how hard it felt they worked on average over the entire workout (including warm up and cooldown).
At the end of a training session, it is up to the athletes to determine where they are on the scale ranging from 0 - 10.
When interpreting the scale, this of it as 0 being nothing, like just floating in the pool. And 10 is the max, like the hardest you have ever worked, you have absolutely zero left in the tank after that workout.
The RPE scale takes into account external stressors in an athlete's life (like sleep, school, relationship, eating) which all impact perception of a workout's difficulty level.
Including RPE in load monitoring provides a more comprehensive view of an athlete's overall well-being.