Module serves as a container of different parts of your app such as controllers, services, filters, directives, etc. Modules can be referenced by other modules through Angular's dependency injection mechanism.
Creating a module:
angular .module('app', );
 passed in above example is the list of modules
app depends on, if there are no dependencies then we pass Empty Array i.e.
Injecting a module as a dependency of another module:
angular.module('app', [ 'app.auth', 'app.dashboard' ]);
Referencing a module: