The Spacing Effect
Spaced repetition is a technique that leverages the psychological spacing effect. The central idea of the spacing effect is that information reviewed a few times, over a longer period of time, is more effectively memorised than the same information reviewed many times, over a shorter period of time. This can explain why cramming before an exam (frequent reviews over a short period) may result in a good short term retention of information, but you forget the information just as quickly.
Spaced repetition uses some of the theory behind the spacing effect create personalised schedules for the information you want to learn.