Your brain forgets — on purpose. The Ebbinghaus forgetting curve shows that most of what you learn today is gone within 48 hours unless you revisit it.
Spaced repetition fights that by re-showing you a fact just before you're about to forget it. Each successful review pushes the next one further out: 1 day, 3 days, 7 days, 16 days.
ExamPilot does this automatically. Every flashcard you make has a review schedule. Open the app, do today's queue, close the app. That's the whole system.
The trick is consistency, not volume. 10 minutes a day for two weeks will beat a single 4-hour Sunday session every single time.