What It Means to Enable A Module
When you enable a module it will show up in the admin area of your site. This is how you can control what you or your client sees. If they are not using a half dozen of the more common modules there is no reason to clutter up their admin with extra links.
How to Enable a Module
To enable a module make a folder in html/mercury/ with the name of the module. If you want to enabled the 'messages' module then you would make a folder at html/mercury/messages. Once you have done this all that remains is to make a file in that folder with the filename of .enabled which can be any size (ie. the contents of this file do not matter, only its presence does). In our example the path to this file will then be html/mercury/messages/.enabled