A module is a self-contained adventure or an entire world, in the case of a PW. They are run as the basis for the environment and contain the rules and details of the content of the place that players adventure within.
Modules can be created by anyone who purchases the NWN game.
If the game is a story, then the module is the book itself. The module contains all the locations, the cast of characters (excepting only the Player Characters), items, objects, and the main plot thread(s).