Spaced repetition (also known as spaced rehearsal, graduated intervals, repetition spacing, repetition scheduling, spaced/expanded retrieval) is a learning method by which you review learned information at gradually increasing intervals. This method uses the spacing effect (sometimes called distributed practice) which increases the recall of learned information when the learning is spaced out over time as opposed to being crammed into one learning session.
For example, you will remember more information if you study for an exam for an hour every day for a week leading up to the exam than if you "crammed" and studied 7 hours the night before an exam. Spaced repetition is when the intervals in between studying would be gradually increased. This type of learning and studying has been shown to be very effective when you need to learn mass amounts of information that you must be able to recall. For example, math formulas or vocabulary for a new language.