NativeScript is a framework that allows building native iOS and Android apps using JavaScript and CSS. It provides a runtime for creating apps with a single JavaScript codebase that can run natively on each platform. Key features include using existing JavaScript and CSS skills, two-way binding, support for Angular 2, and a low learning curve. The document then discusses the NativeScript runtime, popular IDEs and CLI commands, components and layouts, CSS support, debugging, and example apps built with NativeScript.