Anki

Anki is a spaced-repetition flashcard program: cards you find difficult reappear sooner, cards you know well reappear later, so review time concentrates where it is needed.

That scheduling is the whole value, and it is well established — the technique is used heavily in medical education and language learning where large volumes of material must be retained long-term.

It is free on desktop and Android, paid on iOS, and stores decks locally with optional sync. Shared decks exist for many subjects, though making your own cards is generally more effective.