Clustering is easiest to explain with an example. If, for instance, you're in the 5s and get to a weight too heavy to do 3 sets of 5, you might use clustering as follows. Do a first set of 5, rest a bit, do a set of 3, rest again, do 3 more, rest, do a set of 2, rest again and do 2 again. Total reps 15, done when you couldn't do 3 sets of 5. Various combinations of sets/reps/amount of rest are possible.
It's similar to Max-Stim but different in that it seeks to manage fatigue enough to allow completion of the desired number of reps, but Max-Stim seeks (as much as possible) to completely eliminate fatigue.