1. What does it mean?
In casual Korean, 텐션 often describes how lively or excited somebody is. 텐션이 높다 means high energy, while 낮다 suggests a quieter or more subdued mood.
Also written or said as tensyeon, tension
2. The nuance
Do not automatically translate it as English 'tension,' which often suggests stress or conflict. This entry covers the social-energy sense, not technical uses of the loanword. A quiet mood is not a personality flaw.
3. Examples
“새벽인데 다들 텐션이 왜 이렇게 높아?”
It's the middle of the night. Why is everyone so energetic?
“오늘은 텐션이 낮아서 조용히 이야기하고 싶어.”
I'm low-energy today, so I'd like a quiet conversation.
Sources
Nate News: Korean usage in the press
References support the meaning and usage. Example sentences are written for NoCapWiki.