RM=Rep Max
1RM is just a short way of saying 1 Rep Max. Meaning the most weight you can do for 1 rep on a given exercise.
You can have 3RM, 5RM etc. etc.
PR=Personal Record
Most you've ever lifted for X reps on a given exercise.
They sound the same but they can be different. You could currently have 1RM of 300 but a PR of 315, that sort of thing.
Its not a dumb question, I asked it at one point in time