While ReactJS was designed to create online applications that can be accessed via web browsers such as Chrome, Edge or Safari, React Native is used more for mobile application development; note that it's an application framework, not a library of lines of code.
The syntax is the same, but once the project has been completed, React Native can compile your application into native code for iOS or Android, and distribute it via the Apple and Google stores.
So you can write a single version of your code and make it native for both major mobile platforms.
While it is possible to use ReactJS as part of a mobile project, its interface will only be a roundabout way of accessing a traditional web application.
By using native source code for each platform, you'll get a mobile solution of much higher quality, and above all, much better performance.