Drop sets can be a very effective way to stimulate muscle growth, however most trainees do them all wrong. You perform a regular set to muscle failure and then reduce the weight to continue on with more reps (without rest).
While one can perform several drops within one set, most overdo it; they often don’t remember the ideal intensity threshold to stimulate muscle growth: they often work their way down to weights so low it’s ridiculous.
Always remember that growth is stimulated both by load and duration. If you go down in weight too much, the load factor is too low to stimulate growth even if the duration is very long.