1. What does it mean?
ㅂㅂ represents the repeated initial consonant of 바이바이, 'bye-bye.' It commonly appears as a brief sign-off before someone leaves a chat or logs out.
Also written or said as ㅂㅂㅂ, ㅂㅇ, ba-i-ba-i
2. The nuance
It is friendly shorthand in the right group, but can feel abrupt after a serious conversation. Use a fuller goodbye when the relationship or situation calls for it.
3. Examples
“나 배터리 없어서 먼저 나갈게. 다들 ㅂㅂ.”
My battery's dying, so I'll leave first. Bye, everyone.
“얘기 재밌었어. 낼 또 보자, ㅂㅂ.”
That was fun. See you again tomorrow, bye-bye.
Sources
Sinjoeo Korean Slang Dictionary
References support the meaning and usage. Example sentences are written for NoCapWiki.