Anki is an app/website for memorising flash cards. Just like with any flash cards, you have a question or prompt on one side and an answer on the other. A common use for flashcards is having a word in one language on one side and the translation on the other.
What makes Anki different from just using physical flash cards is the system built in for when to re-test you on each card. If you found it easy, you will be tested on that card later; if it was hard, it will come sooner. Over time, this becomes a sophisticated sorting system that matches and reinforces your long-term memory formation patterns.
Using Anki was one of the main reasons (along with mind-maps) that I did well in my exams in school and university. Francis and I still use it now and again to learn language vocabulary. If you are learning anything at the moment, please take a look and see if it would help!