After you memorized a card for the first time, it is presented again on the very next day. If you can still recall it right away it moves to the next stage, which is then shown again in 2 days. Intervals double from then on. If you have difficulties recalling a card, it moves back to the start.

You can customize the intervals yourself per deck to adjust for the subject (e.g. vocabulary cards with only one word per card probably can have longer intervals than, let's say, an anatomy card with details about a muscle, bone or nerve).