In weightlifting, "One-Repetition Maximum" is the maximum weight that you can lift in a single repetition (not more) for a given exercise.
By this number, you can figure out, by using algorithms and calculations how many repetitions and how much weight he should lift.
I did not find the exercise to see Jack's pretests, but if you have his 1RM, you calculate how much weight he should lift in each exercise using the given percentages.