Spaced repetition exploits the spacing effect: memory is stronger when review is distributed over time rather than massed.
The Ebbinghaus forgetting curve shows memory decays predictably; spaced repetition schedules reviews just before forgetting. Each successful review extends the interval until the next.
The failure mode: people use SRS for facts they should understand rather than for facts they genuinely need to recall automatically. Vocabulary, formulas, dates — yes. Concepts that need to be understood contextually — a card won't help.