If a company is developing a mobile app for iOS and Android they may have to create two development teams. One for iOS and one for Android. This is because the two platforms are different enough that engineers will specialize in one. Even though the company is developing one app, they'll technically maintain two. An iOS and an Android app.
React Native is a software development tool, that will let an engineer code one app for iOS and Android. This allows a company to have one React Native development team and build for iOS and Android.
The drawbacks are that the React Native developers don't get direct access to the iOS or Android code, and it can result in performance problems. React Native developers say that the hard parts are when they have to eventually get into the native code for iOS and Android. And from what I heard, that will happen frequently. So it's recommended to develop relationships with Android and iOS developers to help, and you'll eventually need the help.
React Native is still fairly young, and Facebook has been investing in its development. Because development costs are so high, it's worth it for most companies to consider React Native for their mobile development.