The 1RM or one rep max is the maximum weight you can lift in a single repetition for a specific exercise. For example, if the maximum you can lift in a deadlift for one rep is 200 lbs, then this is considered your 1RM for deadlifts.
1RM can be used to determine an individual's maximum strength. It is also used as an upper limit. Helps you determine the desired "load" for an exercise, how much you should lift in other rep ranges. In the app, this is done as a percentage of your 1RM. In other words, if your 1RM is 200 lbs, and the exercise asks for 50% 1RM, you should lift 100 lbs.
The most accurate way to calculate is to actually do a 1RM. However, you can find 1RM calculators online: https://www.bodybuilding.com/fun/other7.htm
Remember: When testing your 1RM ensure you have a spotter, you are in a reliable and consistent testing facility, such as a gym, and that above all you are performing the exercise with proper form. Your safety should be your top priority.