What is 1RM?
1RM is short for One Rep Max. 1RM is a measurement of how much you could lift if you only did 1 single rep with proper form. This value will be higher than how much you actually lifted per weight in your set. It is a measurement of your maximal strength. Hevy calculates an estimate of what your 1RM would be based on what was lifted in a given set.
What is 1RM used for?
1RM is used typically to plan how much weight to use for different kinds of weight training. It is used as a benchmark to calculate percentages from. For example, you may decide to do 85-90% of your 1RM for sets with a low number of reps, or 60-75% for sets with a high number of reps.
You can talk to a personal trainer about the best percentages to use, or do research into the best percentages for different types of weight training, but make sure to keep in mind that personal percentages may be different than what you find online, and it is important to be safe and listen to your body if a percentage seems too high to avoid injury.