If you are working on XAML specially when developing the UI in Windows Phone 8 or Windows 8 App, you would have come across this term.
What are Markup Extensions in XAML?
Markup extensions are the expressions that is surrounded by the curly brackets in XAML. The mark-up extensions are generally used to bind the data or for the resources
An example of the Markup Extensions in XAML is the FontFamily attribute.
FontFamily="{StaticResource PhoneFontFamilyNormal}