A weight lifting term.
A drop set is a set of repetitions that are performed immediately after failure is reached on the main work set. Drop sets are characterized by a work set that is done to failure, then immediately followed up with a lower weight set that is also done to failure.
Drop sets are usually done in increments of 20%.
Working to failure is great, but it does not indicate that you are completely exhausted. A drop set is an effective method of squeezing out some extra reps and keeping the workout intensity high.