Can someone please explain the anatomy of MasterDetails page?

I'm not sure I understand how everything works together.
To start we have a MainPage which inherits from MasterDetailsPage.
And that is the first thing that gets loaded from App.cs

But it displays what looks like a ContentPage. Is the MasterDetailsPage just the part that slides out from the side when you push the hamburger button? Or is it the entire page in the window?

Is there a link to a diagram to what each part is and how they relate? Or can someone sketch it/describe it?

Thanks