Spaced repetition software ("SRS") is essentially digital flashcard software. But unlike other flashcard software, it tries to schedule card reviews at efficient intervals. The theory behind spaced repetition is that humans rarely memorize a new fact all at once. The first time we see a fact, we may only remember for a day or two. But if we force ourselves to remember or review that fact before we forget it, we'll remember it longer the next time. So if we review something at increasing intervals, we'll have a good chance of remembering it indefinitely. It's possible to do this by hand, using index cards and some sort of scheduling system, but most people use special flashcard software with built-in support for scheduling cards.