"평평" is an adverb that describes something coming in abundance (mimetic, onomatopoeic). "오늘 중부지방에는 함박눈이 펑펑 쏟아졌습니다." => Dense snow fell heavily in the central area. 펑펑 may be used like 눈이 펑펑 내리다, 평평 울다(cry uncontrollably), 돈을 펑펑 쓰다 (spend without restraint), etc. In a different use, 펑 can describe something exploding, and 펑펑 multiple explosions. Such adverbs are a class of words very well developed in Korean. They give a vivid, in-the-action kind of description. - 물이 철철 넘친다: Water spills with big splashes. - 나뭇잎이 우수수 떨어졌다: Leaves fell with a rustle. - 기차가 덜컹덜컹 지나갔다: A train went by with a creaky rumble.

펑펑 is a mimetic indicating something happening "explosively".