Most of what we learn fades within days. That sounds discouraging, but it is also the key to studying efficiently: the moment you are about to forget something is the best moment to review it.
Spaced repetition schedules each card at the point where recalling it takes a little effort. That effort is what makes the memory stronger. Review too early and you waste time; too late and you start from scratch.
Lerno uses an open scheduling algorithm called FSRS. It looks at how quickly and accurately you answer each card and estimates when your memory will drop. Cards you know well drift further apart, while tricky ones come back sooner.
The practical takeaway: short daily sessions beat long weekly ones. Ten minutes a day keeps your whole deck alive.