The most common Korean word for “really” is 정말 (jeongmal). There is one more word with a similar meaning — compare them below. 「really」는 한국어로 정말.
really; truly
TOPIK 1 · noun
really; truly; very
TOPIK 1 · adverb
Words and levels: TOPIK I essentials (National Institute of Korean Language standard curriculum, KOGL Type 1) · examples: Cheesepotato