Shortened version of the word Moderators. Mods are usually identified on image boards and forums like 4chan. They are the staff that make sure the forum rules are being followed, and delete threads and ban users that break them. Mods are often despised and hated largely by communities like 4chan, under conviction that they are too strict and that there is too much restriction.
Anon. : Posting NSFW cause FIGHT THE POWAH (USER WAS BANNED FOR THIS POST)
Anon. 2: Fucking mods, they're all fags.