Modules are precompiled source code stored in the 'mod' folder within your BlitzMax installation directory. The entire language of BlitzMax is based on the official modules i.e. those managed by BRL.
A modscope is a reference to a group of modules which are related to each other in some way, usually by author. The modscope is followed by a full stop ('.') and then the module's name. For example, 'Brl.Blitz' is the same as saying, "the module name Blitz from the module scope Brl".