1. What does it mean?
너프 is the Korean borrowing of nerf. It describes reducing a character's, ability's, or item's effectiveness, usually through a balance update. 너프 먹다 means to receive such a reduction.
Also written or said as neopeu
2. The nuance
It is different from a temporary negative status effect during play, which is typically a debuff. A nerfed character is not automatically unusable; the actual changes matter.
3. Examples
“이 기술은 사거리가 줄어드는 너프를 받았어.”
This ability was nerfed by having its range reduced.
“너프 후에도 익숙한 캐릭터라 계속 쓰고 있어.”
Even after the nerf, I keep using the character because I'm familiar with it.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.