1. What does it mean?
ㅇㄷ can abbreviate 어디 when asking someone's location. In comments it can instead mean 와드, borrowed from a game's vision-providing ward: leaving a marker so you can find the post again.
Also written or said as eo-di / wa-deu
2. The nuance
Read the setting before translating it. A comment saying ㅇㄷ does not itself guarantee bookmarking or notifications; those functions depend on the platform.
3. Examples
“나 매표소 앞인데 너 ㅇㄷ?”
I'm by the ticket office. Where are you?
“주말에 다시 읽으려고 이 정리 글에 ㅇㄷ 남겨 둠.”
I'm leaving a marker on this roundup so I can read it again this weekend.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.