While "mod" is often used as a shorthand for "moderation" oder "moderator" on message boards like this one, in Neverwinter Nights (1 & 2) it usually refers to a "module".
A module is, simply put, a part of the game that you play through; an adventure. String several along, and you've got yourself a "campaign".
For example, the NWN2 campaign consists of several modules, each representing an act in the game.
You can find lots of different modules online, created by other users. Ssome are long, some short, some feature lots of battles, some only few etc.
The modular structure of the NWN series is what set it's apart from e.g. the Baldur's Gate games. Especially in how a your character(s) in the game are not really tied to a certain adventure; you can them with you from one module to the next, if you like to.
Or, like the [url='http://nwn2.wikia.com/wiki/Module']wiki[/url] puts it: "Modules are self-contained game files that hold all of the gameplay information for a particular adventure. "
That been any help to you?