The front page is the initial page seen by someone reaching a Moodle site after or before a login. Typically a student will see courses, some blocks of information, displayed in a theme. In the Navigation bar and Navigation block it is called "Home".
A combination of site policies, user authentication and front page settings determine who can get to the front page. And once they get there what they can see and what they can do.
In many ways, a Front Page has similar features and functions to those in a Course. For example, Activities, Resources and Blocks can be added to the Front Page to give it different looks.