React Native one of the Programming language that allow you to build mobile application using JavaScript. Its Also Use same approach as React JS Where it allows developers to design Mobile UI declarative components.

In Simple Word React Native Also use UI building block as you build in IOS & Android. But In react native developer can put those components together with help of JavaScript.

React Native saves lots of time of developers because you don't need to recompile all the data, it allows you to reload mobile application instantly.

Another good thing about using react native is that if your components are written in objective-c, java and swift react native can combine them very easily. You can build part of your app using native code.

React Native is open source framework which can render UI for both android and iOS. It means you can reuse code for building both mobile application android and IOS, there are lots of React Native Developers Chicago who have started using React native for rendering ui for both devices.