What step does the r in prices stand for? why is this step important?

R in PRICES stands for Rest
Rest is important because you need to let the muscle or join injury recuperate and let it be stress free . The more rest you give it the faster the healing process

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)