Spaced repetition is a memory technique that enables you to retain information for long periods of time. The idea is to revise the information at intervals just as you are about to forget it.
At the point in time you learn some information, it is fresh in your mind, and you can remember 100% of it. However, the amount you retain will quickly drop off. Revising the material shortly after would boost your retention back up to 100%.
A rule of thumb is to revise 10 minutes after the learning session, then 24 hours later, then 1 week, then 1 month, then 3 months then 6 months after. The reason for the intervals getting longer is that the more you revise the material, the longer you have before your retention goes back to the same level of forgetting.
Spaced repetition allows you to keep retention high while not spending too much time on revising – you only need to revise four times in the first month! This contrasts with rote learning and cramming which rely on multiple revision sessions in much shorter time frames. This is because spaced repetition takes advantage of the psychological spacing effect, and also leads to better long-term retention.
Information that is usually learnt by rote, such as vocabulary or lists of facts, are well-suited to the spaced repetition technique. Other types of information such as conceptual frameworks are better suited to other techniques such as holistic learning.