1. What does it mean?
공방 abbreviates 공개방송 when fans discuss attending a broadcast featuring an artist. It can refer to the artist's broadcast schedule or the fan activity of joining the audience, subject to the event's rules.
Also written or said as gongbang
2. The nuance
Context matters: 공방 can also mean a craft workshop or an exchange of attacks. Here it is the broadcast-related fandom abbreviation, not a promise that anybody can enter.
3. Examples
“공방 신청 공지가 올라와서 준비물을 확인했어.”
The broadcast-attendance notice went up, so I checked what I needed.
“공방 다녀온 친구가 무대 후기를 들려줬어.”
My friend told me about the performance after attending the broadcast.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.