A repetition maximum (RM) is the the most weight you can lift for a defined number of exercise movements.
A 1 RM, for example, is the heaviest weight you can lift if you give it your maximum effort. A 1RM is your personal weightlifting record for any particular exercise. It could be a squat or deadlift or any other.
A 10RM would be the heaviest weight you could lift for 10 consecutive exercise repetitions.
The exercise program calls for 3 sets of 6RM lifts.