1. What does it mean?
N수생 uses N as a variable number of attempts, extending terms such as 재수생 and 삼수생. In university-admissions conversation it generally includes people attempting admission again after the usual first final-year-of-school attempt.
Also written or said as 엔수생, Nsusaeng
2. The nuance
It does not identify an exact age or count on its own. Avoid assuming why a person is retaking an exam or treating the label as a judgment of their ability.
3. Examples
“스터디에는 고등학생과 N수생이 함께 있어.”
Our study group includes both high-school students and repeat exam candidates.
“N수생이라는 말보다 지금 준비하는 목표에 집중하려고 해.”
I'm trying to focus on my current goal rather than the label of repeat candidate.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.