A card’s retrievability is the probability you’ll be able to successfully recall the answer within a short amount of time, if the card is presented to you for review. Retrievability decreases steadily over time, then jumps up to near 100% again when you recall the answer.
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In spaced repetition practice, we aim to always keep the retrievability of our cards higher than some threshold (often called desired retention or forgetting index), so that we always have a good chance of remembering them when we need them. The amount of practice required increases exponentially as the desired retention approaches 100% – that is, each additional minute we spend practicing boosts our memory less than the minute before did – so most spaced repetition systems aim for a desired retention of 85–90%, which is a good balance between effectiveness and practice time. When a card's estimated retrievability drops below the desired retention threshold, it enters your queue for practice.
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An important but poorly understood point is that your retrievability when recalling a random card at a random time – like when you need it in real life or on an exam – is significantly higher than the desired retention, as long as you stay more or less up to date on practicing your cards. That’s because the moment the scheduler puts a card in your practice queue is the moment you know it worst: your memory is now bad enough that you need a refresher, and once you get it, your memory will be much better again. A randomly selected card at a random time will, on average, be some time away from its next review, and thus have a higher retrievability. The math of actual moment-in-time retrievability is too complex for this article, but a good rule of thumb is that if your target retrievability is 90%, your actual retrievability on a random card at a random time is 95%.
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A card’s stability is how firmly the idea is stored in your memory. The higher a card’s stability, the slower its retrievability decreases over time. The more times you successfully recall a card, the higher its stability. Stability corresponds to a card’s interval parameter in a spaced repetition system, i.e., the number of days before the retrievability drops below some desired value.

Final Review Period (temporary increase in retrievability): On your exam date, you want your retrievability as high as possible. Maintaining, say, 98% constantly is impractical, but doing extra practice to reach a very high retrievability for the day of your exam is easy enough once you have a solid understanding of the cards. When the Final Review Period is enabled, we add one last review of every card just before your exam.

Learning Period: Brand-new cards are forgotten fastest, since there's no stability behind them yet. During the Learning Period, a new card is repeated until you've recalled it successfully twice, which brings your retrievability back up near 100% before the main scheduler starts spacing it out over days. This step guarantees you fully comprehend the ideas before the algorithm begins testing your desired retention over longer intervals.

Ensure Mastery: The cards you keep forgetting are the ones most likely to fail you on exam day. When you turn on *Add extra practice for shaky cards*, a card you get wrong doesn't simply return to the normal rotation: the scheduler brings it back sooner and asks for two successful recalls in a row before treating it as learned again. Your shakiest material gets concentrated practice, while cards you already know well are left alone.

Catch Up Period: If you fall behind, the exam scheduler proposes an explicit Catch Up Period (a temporary increase to your daily goal) rather than quietly rearranging your schedule. By temporarily increasing your daily review goals, this window forces you to practice a higher volume of cards whose estimated retrievability has dropped below your target threshold.

Exam Daily Goal: Instead of rescheduling cards in the background, the scheduler computes a daily goal for your exam from the practice you have coming up this week and the total practice remaining, and shows it as its own section in your daily goal. ​