The NativeScript technology can be presented as built upon several major parts - Runtimes, Core Modules, CLI, Plugins. Consider the following diagram:
Another thing the Core Modules provide is a basic XML-based way for defining UI, data-binding and navigation. Along with it, Angular and Vue.js can be used as Application Framework technologies. Read more in the Modules documentation.
It is the command-line interface that lets you create, build, and run apps using NativeScript. The CLI runs on Windows, macOS or Linux, and you can find more information in the NativeScript CLI repo.