1. What does it mean?
ㅎㅇ keeps the initial consonants of 하이, the Korean spelling of English 'hi.' It is a quick greeting when joining a casual chat or recognizing a friend online.
Also written or said as ha-i
2. The nuance
It is written shorthand, not a formal greeting. 안녕하세요 is a better opening with a teacher, customer, or someone you do not know well.
3. Examples
“다들 ㅎㅇ, 나도 이제 접속했어.”
Hi everyone, I've just logged in too.
“민지야 ㅎㅇ, 오늘도 그림 그리고 있어?”
Hi Minji, are you drawing again today?
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.