The 21 is a technique commonly used in curl for biceps, although it is possible to perform in more exercises.
Here is the example of a set of 21's for the bicep curl exercise:
7 bicep curl repeats with the first half of the movement (from bottom to middle)
7 bicep curl repeats with the second half of the movement (from half to above)
7 complete bicep curl repeats (normal, total movement)
All this was done followed without rest, totaling a total of 21 repetitions.