And now I'm going to add it to my queue for making Anki cards.
When you review a card, you can mark it as "Again", "Hard", "Good", or "Easy". The new interval of a card will depend on the grade, so it might be like 1day -> 2days / 3days / 4days, and so on. "Again" resets the interval to 0, like a new card.
The documentation is "open source tier", because anki is open source. It should be better, I agree, but something something you get what you pay for. Anki is free on pc/mac and android, but not on ios.
As someone who has used it for the last 2.5 years, I can wholeheartedly recommend it. In fact, I've written at length about it [2]. However I think the manual is pretty good [3] (for anyone who's curious).
Feel free to ask if you have any specific questions.
[0]: https://s3.amazonaws.com/andrewzah.com/studies/Pashler.Rohre...
[1]: https://s3.amazonaws.com/andrewzah.com/studies/Bahrick-et-al...
Anecdotally, it's also a lot more engaging than your average flashcard app. Using Anki feels like playing a video game, except that you also get quite valuable results out of it.