React Native is an open SOURCE framework developed by Facebook which enables developers to BUILD cross-platform mobile applications using Javascript. With React Native, one can develop a mobile application by using the same design principles used to develop a web application with ReactJs framework. It allows the developer to build mobile application UI by composing MULTIPLE components in a declarative way. Before React native, there were few OPTIONS like Cordova, ionic available to build a hybrid application. 

These applications were written using web technology but the hybrid app was not a native application and lacks performance issue. React native solves those performance issues that is why it quickly became popular in React community. Under the hood, React native bridge invokes the native rendering APIs in Objective-C (for IOS) and Java (for Android). That is why they perform better than hybrid application development frameworks. React native has a very good community of developers who ACTIVELY contribute to the framework. Its code is available on Github and has 70k+ stars.