A drop set is basically an extended set of a move, usually performed as the last set of that exercise as a burnout. For example, for a seated dumbbell shoulder press, you’d do two sets of 10 to 12 reps using a certain weight.

Drop setting involves performing an exercise with a specific amount of weight to start. The goal is to do as many reps as possible until momentary fatigue (unable to complete another single rep), rest just long enough to remove some weight (dropping the weight) and then continue to work until another moment of fatigue.