A more complete answer. It's a useful app for spaced repetition. Spaced repetition is a way of scheduling your practice/study sessions that aims for optimal recall. We all have a forgetting curve. You read 100 facts today and tomorrow you will only remember 80. In a week you may still remember 50. At the end of the year you'll be doing good to remember 20. (this assumes no further reviews)
SRS (with Anki or not) schedules reviews to try to help you remember. You study 100 facts today. Tomorrow you review them all (this is actually a bit much for someone just starting, but go with it). You get 50 correct, you'll see them again in 2 days. You get 50 wrong, you'll see them again the next day. Eventually, everything you've studied for a long time will either be scheduled for reviews way far out in the future, or you'll be reviewing it frequently (which often means the card is too hard or you otherwise just aren't getting it). You edit the card to break it down ("Columbus sailed on the Pinta, Santa Maria, and Niña in 1492 from Spain.", that's at least 6 facts: name, ships, year, origin).
This is better than the alternative, and the way many of us (or at least my classmates and I) did it in school. We made flashcards and just reviewed all of them over and over again. It was a long slog to get through, and often we'd just skip it rather than deal with it. Or the ones at the bottom of the stack would never get reviewed properly. SRS works in much the same way, but you don't review the full deck each day or even each week. If you learn a fact really well, you won't see it again for months. If you still know it then, you won't see it again until the next year. So the only things you study regularly are new things, or things that aren't (for some reason) sticking.
Anki is a useful (and free) application that will sync across your computers and devices (I make cards on my laptop, but review them on my phone) that implements SRS. You can, through plugins, do a lot of neat things like autogenerate math problems so you aren't practicing the same ones over and over (and risk just memorizing them rather than learning the techniques), or have it render LaTeX to the card. It'll show images, play audio, and more.
Accumulate knowledge in intervals?