Spaced Repetition is a learning technique typically used to memorize flashcards, but it can be applied to other study material as well.
The idea is to review your flash cards at increasing intervals. The more confident you are about a card, the longer you can wait before you review the card again.
Let’s say you are reviewing a card that you already recalled correctly 5 times. There’s no need to review that card again tomorrow. Instead, you can look at it again next week. If you still remember it next week, try again 2 weeks after that, then 4 weeks and so on.
The interval between reviews (i.e. the number of days you wait until you look at a card again) increases over time so that you see your cards less and less often. The ideal time to review a card is just when you are about to forget it. Studies have shown that recalling a fact at that time makes you memorize it even better.
Should you review a card and realize that you forgot the fact on the card, that card goes back to square one and is reviewed again the next day. Cards that you forgot are reviewed more often and intervals increase again as you become more and more confident about a card.