I first heard about SRSs (Spaced Repetition Systems) many years ago when I was learning Mandarin and I was trying to find a reliable way of learning hanzi.
So an SRS is a computer program that helps you learn facts in flashcards efficiently over time by having an algorithm choose for you what to review based on when you are likely to forget it. This way, you don't waste time trying to study material that you already know pretty well. When you review a flashcard, the SRS prompts you to evaluate yourself on how well you know it so that it can calculate when to next schedule the flashcard.
The first SRS that I used was Supermemo. There were not as many decks (a collection of flashcards) available today compared to today, so for my first deck, I went through an entire Chinese dictionary and created a short flashcard for each (character in the front, pronunciation in pinyin in the back). Looking back, I was pretty hardcore.
Later on, I moved on to Anki, another SRS. Over time, more decks became available online and I started downloading them for the most part.