21-15-9

21-15-9 is three reps rounds, where the first round is 21 reps, the second round is 15 reps, followed by the third round with 9 reps.  If there are multiple exercises you should complete each of the exercises per the reps in that round, before dropping down to the next next rep round and repeating each of the exercises per the reps in that next round, and so on.