1. What does it mean?
As a reply, ㅇㅎ often represents 아하: the moment you understand an explanation or connect the dots. It is a short acknowledgment rather than an elaborate answer.
Also written or said as a-ha
2. The nuance
The same initials can be a suggestive-content warning in some post titles. Do not read every ㅇㅎ as 'aha'; its position and the surrounding topic matter.
3. Examples
“ㅇㅎ, 입구가 건물 뒤쪽에 있었구나.”
Oh, I see, the entrance was behind the building.
“ㅇㅎ 이제 이해했어. 먼저 접고 붙이는 거네.”
Aha, I get it now. You fold it first and then glue it.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.