1. What does it mean?
ㅉㅉ represents 쯧쯧, a repeated tongue-click. In comments it commonly conveys that the writer disapproves of a choice or looks down on what happened.
Also written or said as jjeut-jjeut
2. The nuance
It can feel openly contemptuous even without a written insult. It is not laughter; avoid sending it to someone seeking comfort unless you understand how it will be received.
3. Examples
“실수 얘기했더니 ㅉㅉ만 보내서 기분 나빴어.”
I mentioned a mistake, and their reply was just a disapproving tsk-tsk. It felt bad.
“그런 일로 사람한테 ㅉㅉ거릴 필요는 없잖아.”
There's no need to tsk-tsk at someone over that.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.