Spaced Repetition
A learning technique that incorporates increasing intervals of time between subsequent reviews of previously learned material in order to exploit the psychological spacing effect.
Spaced repetition is the most efficient way to commit information to long-term memory. It combats the "Forgetting Curve" by scheduling reviews just as you are about to forget the information.
The Forgetting Curve
Hermann Ebbinghaus discovered that memory follows an exponential decay curve. Reviewing material interrupts this decay and flattens the curve, meaning you remember it for longer periods after each successful review.
Implementation
Modern implementation relies on algorithms (like the SM-2 algorithm used in Anki) to calculate the optimal review date for each card based on your past performance.