Ah, Anki. What is left to say about Anki? For those of you unaware, it might not be clear why Anki, a simple flashcard application that uses a spaced repetition system to help improve your learning ability has attracted both such a devoted following and reputation for unintuitiveness.
As mentioned previously, Anki is a free, open source flash card application available on pretty much every platform. You can create flashcards which are stored in virtual decks, or download publicly shared decks available online. When you review a card, you put some info on the front and back, and then grade yourself on how well you were able to recall this information from four different options. Depending on what option you choose, Anki's spaced repetition system algorithm will adjust the amount of time until it asks you to review and recall that same information, ideally just before you'd forget it normally. Because of this, it's an extremely powerful tool to memorize information, and has become widely used in the language learning community particularly for memorizing vocabulary.
Really what makes Anki so powerful is how flexible it is. There is a whole ecosystem of add-ons available, both free and paid, as well as thriving communities based around its use for learning Japanese, especially for what's called "sentence mining," where you study single words in the context of a sentence pulled (or "mined") from some native material.
Everything in Anki can be customized, from the look of the cards, to what information you put on it, to how the cards are generated and organized, and even the algorithm it uses itself. However, this power also adds a layer of complexity that can make starting to use Anki seem formidable. Luckily, there are tons of guides available now on configuring Anki and its use depending on your intention, as well as hundreds of pre-made decks if you just want to dive in. Anki can be an extremely powerful tool to level-up your Japanese, but it's not without its difficulties.