This is a learning method that improves recall and typically done with flashcards. This is based on the idea of a 'forgetting curve' - the rate at which we forget new information over time. If we are exposed to the information again at the main points of the forgetting curve, we will not forget it.
The system shows us flashcards with a question. If we answer it correctly, that card goes into a 'review later' pile. If your answer was wrong, it goes back into the 'review next' pile. So whichever things you got wrong will be asked again frequently. Whatever you got correct, will only be asked again later.