React Native is a library built with the same API as React. It uses the same declarative UI paradigm but renders directly to native components. You can write your app once in React and run it on multiple native platforms, like Android and iOS.
Note: Without Expo or React Native, pure native app development would require you to learn the platform language and tooling like Android Studio for Android and Xcode for iOS.
By using React, you can reuse prior web development knowledge to create mobile with the native features of traditional mobile apps. It also allows developers to share most of the code on all platforms, resulting in faster development. But, to get from idea to app, basic knowledge of the native platforms is required.