1. What does it mean?
인정 is an ordinary word for acknowledgment or recognition that also works as a short casual response. On its own, it signals that you accept a point or agree with someone's assessment.
Also written or said as ㅇㅈ, injeong
2. The nuance
This response use is not automatically an admission of wrongdoing. Bare 인정 can sound clipped in a formal exchange; 맞아요 or 동의해요 makes the agreement more explicit and polite.
3. Examples
“이 길은 조금 돌아가도 조용해서 좋다. 그건 인정.”
This route is a little longer but nicely quiet. I'll give you that.
“이번 표지 색 조합 좋다는 말에 나도 인정.”
I agree that the color combination on this cover looks good.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.