The Leitner System is a spaced repetition technique for learning with flashcards.
Cards are sorted into boxes based on how well the material is known.
Correctly answered cards progress to boxes that you don't need to review as often, while incorrect ones are moved back to the first box for more frequent review.
It turns the learning process into a kind of game where the aim is to have your flashcards as far along the chain of boxes as possible.