Expanding intervals and the forgetting curve
Spaced repetition schedules reviews at intervals that grow each time you successfully recall an item: a first review after a day, then a few days, then a week, then weeks, then months. The intervals expand because each successful retrieval strengthens the memory and slows the rate of forgetting.
The idea traces to Hermann Ebbinghaus's forgetting curve, which showed memory decays rapidly at first and then levels off. Reviewing just before predicted forgetting resets the curve more efficiently than massed cramming.
How algorithms set the next interval
Modern flashcard systems automate the schedule. The SM-2 algorithm popularized by SuperMemo, and used in tools like Anki, lengthens the interval after a correct recall and shortens it after a lapse, adjusting an ease factor per card. The result is that easy material is shown rarely and difficult material more often, concentrating study time where it is needed.