It is the go-to app for free (except on iOS) Spaced Repetition System (SRS) flashcards. It has a simple user interface with various features that more hard-core users can dive into if they choose.
Your flashcards will appear according to your natural forgetting curve; the app will test you in increasingly spaced out intervals, with more difficult cards appearing more than once in a session, while easier cards spacing out over weeks — or even months and years. An SRS system is the most effective way to drive information into your long-term memory.
The cards can sync between the web, desktop app, and mobile versions to keep your flashcards updated and with you at all times. You can add images and audio clips to your cards and change the text formatting (if you use it on your computer). One feature unique to Anki, as opposed to other SRS flashcard apps, is the “Cloze deletion” function, which allows you to block out parts of your card and create a “fill-in-the-blanks” type card format.
If you want a resource for how to make effective flashcards, check out the book, FluentForever. The author leaves a whole section dedicated to understanding how to use your Anki deck to advance your skills quickly.