What is spaced repetition, in one sentence?

Reviewing material at increasing time intervals, each review timed to catch you just before you'd forget.

Instead of cramming everything at once, you revisit each item after a gap — then after a longer gap, then a longer one still. Every review that succeeds on time resets the clock and makes forgetting slower next time, so the intervals can safely keep growing.

The pay-off is efficiency: you spend the fewest possible reviews to keep a fact retrievable, because you only revisit it right when it's about to slip away rather than while it's still fresh.