As a Silverlight, WPF or WPDev you might be aware of the MessageBox class present in the SDK to show a message to the user and ask the user to interact with the application on some special cases. As a Win8Dev, you will find it missing in WinRT.
What to do? There is a class named MessageDialog which does the same thing but asynchronously in Windows 8 applications. If you are new to Win8Dev, you will find it useful.