1. 썸타다 썸 is a konglish word from english word'something'. Shortly, it's a middle stage between friendship and relationship. before relationship, the stage of knowing each other by pushing and pulling. If a guy and a girl push and pull one another, Korean people describe the situation like "쟤네 사이에 뭔가 있어~!"(There is a something between them~!) And the description became 썸을 타다 or 썸타다 '타다' is just like 'go along' and 'go with the flow' 2. 뭐가 무서워 If it is translated literally, it's 'What's scary' as you told. Just on the lyrics, the nuance is more like 'It's not scary, so don't be scared' For example, when a korean couple is watching a horror movie, girl is scared of it. The guy tells her like "에이~뭐가 무서워!"(It means 'hey~ it's not scary at all!')

It's a current slang in Korea. "썸을 타다" means having "some" relationships with whom I'd like to love. 한국어로 설명하자면, 연애를 시작하기 전 지속적인 만남이 있는 상태를 "썸을 타다"라고 합니다. "뭐가 무서워"를 올바로 번역하면 What's scary 가 맞습니다. 숨겨진 뜻이 Don't be scared. 겠지요. 영어로도 거의 같은 뜻 아닐까요? What's scary? don't be scared!