A premade deck is a compilation of flashcards put together by somebody else and shared on the Internet.
There are certain premade decks that are designed to teach you the most common basic words and grammar of a language starting from zero, via sentence cards, so you could use one of these if one is available for your language.
We have two basic requirements for using a premade deck to get started:
There are also decks that are not for beginners, but you should never use these. The information will never really stick.
The first reason is that it's hard to judge which premade decks are good if you're a beginner.
The other reason is that it seems there are hardly any decks that fulfill our two basic requirements, as stated earlier: (1) it should only teach via sentences, and (2) it should be designed for people who have zero prior experience with a language. I realized this because I've also dug into premade decks for other languages for friends interested in this approach (and, admittedly, out of personal curiosity).
If you're learning Korean, I can personally recommend Evita's Grammar deck (do not use his Vocabulary deck - only study sentences). It isn't perfect but it's like 95% fine.
If you're learning Japanese, I have heard good things about this deck based on Tae Kim's guide.