1. What does it mean?
앙콘 shortens 앙코르 콘서트. Fans use it for an extra concert or return run following an earlier series or tour. It generally means a separately scheduled event rather than a few extra songs at the end of one show.
Also written or said as angkon
2. The nuance
Do not confuse a concert's encore segment with a new 앙콘 date. Ticketing, setlists, and eligibility depend on the separate announcement.
3. Examples
“투어가 끝난 줄 알았는데 앙콘 공지가 떴어.”
I thought the tour was over, but an encore concert was announced.
“앙콘 날짜는 지난 공연과 다른 달이야.”
The encore concert is in a different month from the earlier show.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.