Spaced Repetition is a studying technique that increases the time in-between review sessions.
In practice, if you reviewed a topic a day ago, then you should review it again after 2 days, and then after a week, and then after a month, and then after three months etc.
The idea is to always increase the time between the reviews in order to review the topic just before forgetting it_.
The actual time in between does’t have to be so specific, and can vary from person to person.
Anki is one of the most popular Spaced Repetition System which uses flashcards to isolate single atomic elements, and depending on how you think you recall them, the time for the next review will vary.