Words in Korean aren't much more than just syllables written together.
Here's a word:
It means "mom".
We can break it down like always:
엄 = ㅇ + ㅓ + ㅁ = nothing + u + m
마 = ㅁ + ㅏ = m + aa
So, 엄마 = umma.
Sentences are obviously just words in order, so:
You can break it down on your own, but we'll just skip to combining words:
넌 = nun
멍청해 = 멍 + 청 + 해 = mung + chung + heh
So, the whole sentence is:
It means, "you are dumb".
Now we'll practice with some popular lyrics:
내 말 맞다면 throw your hands up
거부할 수 없다고 baby girl, oh (Yeah, yeah, yeah)
NCT 127. (2021). Sticker [Lyrics]. Retrieved from https://genius.com/Nct-127-sticker-lyrics