Descending sets are preformed when the weight of a barbell or dumbell are lowered to extened the duration of a given set.
Example:
Dumbell Curls
40lb x 10 reps -> 30lb x 8 reps -> 20lb x 6 reps
40 pounds is used on the intial set when faluire is reach 30 pounds is then used and so on down to 20 pounds. This is a descending set.