Well, after doing disc 1, I think I finally know what these are!
It's a set of 21 reps for one exercise, for example, standing bicep curls, but in the dvd, we did 7 reps lifting only half way up and down, then you bring the barbell to the top of the rep and do 7 reps but from top to the midway point then back up again, then we did 7 full reps.