ReactJS already understands what the application is supposed to do. But what really differentiates React Native from the ReactJS is rendering other than Browser DOM. In React Native, instead of rendering to the browser’s DOM, React Native invokes Objective-C API to render iOS components, or Java API to render Android components.
This is possible because of the bridge, which provides ReactJS with an interface into the host platform’s native UI elements. React components return markup from their render function, which describes how they should look. For React Native, this markup is translated to suit the host platform, so a <View> might become an iOS-specific UIView.