하트뿅뿅 is used to express “emotion of love”.
Literally, 하트 means an English word — “heart” and 뿅뿅 is the word to describe the sound which imitates something flies quickly. So, 하트뿅뿅 describes that hearts pop up from eyes.
This word is not used for serious purpose.
Definition in Korean (뜻)
이 단어는 사랑의 감정을 가볍게 표현할때 사용된다.