Every app consists of one or more windows with one or more pages.
A new Blank App template project is given a single window with a single page called MainPage. It defines what the user sees once your app has loaded and the splash screen has gone away. MainPage, like any page that would be used in an app, is implemented across two files:
MainPage.xaml contains the user interface
and MainPage.xaml.cs contains the logic / code-behind.