쿵짝 is onomatopoeia word. It can be used seperately. 쿵 is kinda big and heavy sound like big giant's walking or instruments sound like a trombone. 짝 is sound of a clap or slap on the face. but if you heard this in the song, it could be used for beating time. 쿵짝 sounds cheerful and lively.

That kind of words are called '의성어(echoic words)'. It's probably "쿵짝쿵짝" in lyrics. "쿵짝쿵짝" is the sound instruments(especially the drums) make, when singing. (to get the beat in excitement)

쿵짝쿵짝 쿵짜라쿵짝 네박자속에.. it imitates drum sound in the song