찰떡-> 잘 맞다 / 딱 맞다
It is along the lines of “to fit well” “to fit perfectly” “be right for”
I saw it several times on instagram, I was confused at first until I realized they were using BTS choreography (thank you commenters lol) for a different song
So they were saying that the choreography went really well with this other song. It fit the song perfectly.
안무 왜 찰떡?? -> how does this choreo fit so well??
이 노래 안무 찰떡이쟈나 ->this choreo fits the song so well! (이쟈나 is for 이잖아)
세팅펌 긴머리에 찰떡이다! This setting perm is perfect for long hair! (-> an article title on naver)
개떡 is the opposite
It is used to describe things as being not right or having gone wrong
개떡 같은 인생 -> a life where nothing works out
(it was defined as 인생이 뭘 해도 안풀린다 -> even if you do something, life goes wrong / life doesnt work out, no matter what you do)
개떡 같이 말해도 찰떡 같이 알아듣는다! -> Even if I say it wrong, you understand right / Even if I am not clear or don’t explain it properly, you know what I mean / you understand me anyways! (from here)
But in a literal sense, both of these are just types of rice cake lol.
UPDATE: I just came across this which has more detailed answers 🙂