What Is React Native, Building With React Native

TL;DR

React Native - Simply an Open Source framework (React) used to create applications that run on multiple devices (Android, Web, iOS ...) using JavaScript.

React Native Docs

To get started with React Native, it is advisable to have basic knowledge of javascript (es2015) and React components and props, just the basics of all these will set you ready.

Native Components: Components that can be used in iOS and Android devices.

Core Components: Ready to use React Native Components -

native and core components

List of core components

React Native Developer Environment

Docs