This is a great set/rep scheme for size gains. It's a scheme popularized by Vince Gironda, and one which he used with the very first Mr. Olympia, Larry Scott, and many others.
It's based on a high training density (doing a lot of work in a shorter amount of time), rather than load.
To do it, you perform 6 sets of 6 reps with a moderate weight that you could do for 10 reps. What about the weight? Use approximately 70% of your 1RM for the particular exercise you're doing, NOT your 6-rep max.
You must complete all six sets in as little time as possible, resting only 30 seconds between sets. That's where the training density comes in. While Gironda's recommendations were to shoot for rest periods of 30 seconds at most, he had his advanced bodybuilders resting only 15 seconds between sets.
Remember, the key factor with this loading scheme is density, not load. If you can't do all 6 sets with a strict 30 second break, reduce the weight until you can use shorter rest periods.