Traditionally, in sets of 21, you do not perform an exercise over the full range of motion in each repetition, but divide it into three sections, each of which you complete seven repetitions. For the first seven repetitions, you only stay in the stretched position until half of the full range of motion. For the second seven reps, move the weight from the half-way point to the peak contraction position. Then follow up with seven repetitions over the entire range of motion, or as many as you can until failure; for a total of 21 reps or more.
Popularized by Ronnie Coleman, the classic exercises for 21s are barbell and dumbbell curls. As a rule of thumb, one can say that all exercises are suitable. This applies to most isolation and cable exercises, but also to squats, bench or shoulder presses. However, back exercises are, due to the pulling action, not really suitable, because the weight has to be significantly lower for you to be able to control the weight the intended way. A real insider tip is 21s for the calves.
Do not commit the mistake of using too much weight. Most athletes overestimate their power with this technique, and then start cheating to get the required repetition numbers. Instead, reduce the weight at the beginning and try to concentrate on the technique and the range of motion. It does not matter if you make 7, 6 or 8 repetitions in the sets. The 21 repetitions are not a dogma.