Every 2 minutes you perform a cluster of exercises based on the goals of the day, being strength, speed, etc. The cluster could be 1 rep, 3 reps, 5 reps, etc with a short rest between each cluster, repeat a few times, then rest longer before the next cluster. Pretty simple. Instead of doing a straight set of 12, for example, you would do 4 clusters of 3 reps as one set. Its still 12 reps, but you can use much heavier weight. Some people like it, some don't, but you'll never know how you feel about it without trying it out. The following video describes one particular way to use clusters with squats, but the options are endless. You can play with the sets, reps, and rest periods to get yourself stronger and better conditioned.