What FSRS actually does

FSRS models two things about each card: how hard it is for you, and how long the memory currently lasts. Every time you rate a card, both are updated, and the next review is scheduled for roughly the point where you would be about to forget it.

That is why intervals grow so fast. A card you keep getting right does not need daily attention, and reviewing it daily is wasted effort that crowds out cards you are actually failing.

What the four grades mean

• Again: you did not recall it. The card comes back in this session.
• Hard: you got there, but it was a struggle. Shorter interval than Good.
• Good: recalled with normal effort. This is the honest default.
• Easy: instant and effortless. Use it sparingly, or intervals overshoot.

FSRS is free on every card. Custom FSRS tuning is a Plus feature for people who want to adjust the parameters.