XML to define interface (converted to native GUI controls by runtime),
and CSS for styling the GUI (again converted to native GUI)
Portable runtimes are available for iOS, Android and Windows,
Command line tools are free, and Telerik provides many online services to help testing, deployment, etc.
Claimed tested performance is around 7% slower than pure "native" Swift code,
and that is similar to Xamarin. Runtime included with each app is a few MB.
Other similar platforms include Appcelerator Titanium and React Native
podcast interview: Understanding NativeScript with Sam Basu @ .NET Rocks!