Your 1RM for a given exercise is the heaviest weight you can lift at least once for this exercise. Studies on strength will often express a weight either as an “X”RM (where X is the maximum number of reps you can reach with that weight) or as a percentage of 1RM.
1RM. The maximum weight you can lift.
25% of 1RM. If your 1RM is 80 kg, then 25% of your 1RM is 20 kg.
15RM. The maximum weight you can lift a maximum of 15 times.