Expo apps are React Native apps which contain the Expo SDK. The SDK is a native-and-JS library which provides access to the device’s system functionality (things like the camera, contacts, local storage, and other hardware). That means you don’t need to use Xcode or Android Studio, or write any native code.
Expo also provides services which typically are a pain to manage but are required by almost every app. Most popular among these: Expo can manage your Assets for you, it can take care of Push Notifications for you, and it can build native binaries which are ready to deploy to the app store.
Feel free to reach out with any questions, ideas, or even just to say hello!