1. What does it mean?
썸 refers to the ambiguous stage when two people seem interested in each other but are not clearly an established couple. The common phrase 썸을 타다 describes being in that stage.
Also written or said as 썸타다, 썸을 타다, some, sseom
2. The nuance
It is more specific than any friendly interaction, but it does not establish commitment, exclusivity, or mutual agreement. One person's sense of 썸 may not match the other's. Ask about the relationship rather than treating signals as a settled label.
3. Examples
“같이 자주 만나는데 사귀는 건 아니고 아직 썸이야.”
We see each other often, but we're not officially dating; there's still something undefined between us.
“나는 썸인 줄 알았는데 상대는 그냥 친구라고 생각했대.”
I thought there was something romantic going on, but the other person saw us as friends.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.