There's a lot of information about how and why SuperMemo works on the SuperMemo website. Here are the two most important articles to get you started:

• A Formula for Success in Learning-- 6 steps to effective memorization
• The Principles of SuperMemo-- why spaced repetitions work

I certainly can't do it better than the software's creator so take a look at the official website and come back here when you're convinced and ready to use it to study Japanese.