1. Pick a weight you can maximally lift 1-12 times.
2. Lift the weight for as many repetitions possible and enter weight and number of repetitions.
3. The top panel shows the estimated RM-values. 1 RM is the maximal weight you can lift 1 time. 2 RM is the maximal weight that can be lifted 2 times, etc.
One RM is defined as the maximal weight that can be liftet once with good form in a given exercise. Two RM is the maximal weight that can be lifted two times etc. By knowing the maximal weight you can lift a given number of repetitions, you can estimate the maximal weights that can be lifted any other number of repetitons (within limits).
Brzycki, M. Strength testing: Predicting a one-rep max from a reps-to-fatigue
Journal of Physical Education, Recreation and Dance 64 (1): 88-90, 1993.