Honestly, I’m just like you; I see everyone mention it and I don’t know what it means.
From context, I can guess an MOC is an outline of content, or a way to organize files? Like, the Waypoint plug-in generates an “MOC” note for each folder, and it creates a list of links to every file inside the folder.
Oh! Map of Content. It could stand for Map of Content, maybe.
But really, I think it’s just a file or multiple files for organization purposes. Like a replacement for folders, I’m guessing.