Suppose there are 3 boxes of cards called "Box 1", "Box 2" and "Box 3". The cards in Box 1 are the ones that the learner often makes mistakes with, and Box 3 contains the cards that they know very well. They might choose to study the Box 1 cards once a day, Box 2 every 3 days, and the Box 3 cards every 5 days. If they look at a card in Box 1 and get the correct answer, they "promote" it to Box 2. A correct answer with a card in Box 2 "promotes" that card to Box 3. If they make a mistake with a card in Box 2 or Box 3, it gets "demoted" to the first box, which forces the learner to study that card more often.
Source: https://en.wikipedia.org/wiki/Leitner_system