What is React Native?
React Native is a framework that allows you to build mobile applications using JavaScript (React).
Okay but what is Expo and why should I use it?
In Expo's own words:
Expo is an open-source framework for apps that run natively on Android, iOS, and the web. Expo brings together the best of mobile and the web and enables many important features for building and scaling an app.
The expo npm package enables a suite of incredible features for React Native apps. The expo package can be installed in nearly any React Native project.