With the Leitner System and spaced repetition, you practice items you haven’t learned more frequently than those you have already memorized. For example, when studying vocabulary words with a flash card system like Anki,
Words answered correctly by a student can be studied less frequently, and incorrectly answered vocabulary would be assigned less priority.
There’s a great article on Medium that covers spaced repetition in regards to using Anki flash cards here: https://medium.com/swlh/level-up-your-learning-with-anki-and-spaced-repetition-3c1a84003ced
Based on how well you’re answering certain questions, Anki automatically tracks how frequently you should review certain flash cards.
Wikipedia covers the Leitner System here: https://en.wikipedia.org/wiki/Leitner_system