“base” is an instance of the ShowBase class. In the ShowBase constructor, it will assign itself to a builtin called “base”, which is from that moment on available from all other modules.
You may not see a ShowBase instantiation if your application imports the direct.directbase.DirectStart module. All this module does is just instantiate ShowBase, which also puts “base” into the builtin scope.
Edit: d’aw, wezu beat me to it!