Variable sets are made up of reps with different points of emphasis. For example, if you are bench pressing a set for 12 reps, with variable reps you might change how the reps are performed, say every four reps. The first four reps might be performed with maximal concentric intent and normal lowering speed, the next four reps are performed with an isometric at the bottom portion and the last four reps are performed with a 4 second lowering. The purpose of variable reps is to challenge the motor system and muscle recruitment patterns within a given set in order to hopefully get more out of the workout, or to work on specific qualities that you are trying to improve upon.
Here’s an idea for the bench press:
3 sets x 12 reps - Reps 1-4 (Lower weight normal speed, lift explosively), Reps 5-8 (Lower at normal speed, hold for 3 seconds, lift explosively), Reps 9-12 (Lower for 4 seconds, lift as explosively as possible)
Try it out. You’ll probably want to use less weight than you are accustomed to for 12 reps, as the level of difficulty is higher than if you did 12 traditional reps. This is an example of progress even with the same amount of weight and reps, simply because you made the reps harder. Get ready to be sore the next day!