ㄴ가

builds "some-" words: something, someone

Buried inside a small set of very common indefinite words, where it turns a question word into a vague one: 뭔가 is "something" or "somehow", 누군가 is "someone", 어딘가 is "somewhere", 무언가 is a fuller "something". You will not see it as a separate word — it is fused into these forms. 뭔가 in particular does far more work than "something": dropped in front of a description it means "kind of" or "sort of", as in 뭔가 클린걸 느낌으로, "kind of a clean-girl vibe". It is a filler people reach for constantly while searching for the right word.

Register: very common in casual speech