Why spaced repetition works
You forget things on a predictable curve. Reviewing a fact just as you're about to forget it strengthens the memory most. In the Leitner system, cards you get right move up a box and come back less often; cards you miss go back to box 1 and come back soon.
Tip: write cards as questions ("What does an enzyme do?") rather than single words, and keep each card to one fact.