I would say, React Native provides low level APIs (Unix philosophy) which can be used to build complex apps. NativeBase uses these low level APIs and provides a set of generic re-usable components for speedy development and prototyping. For example, React Native comes with TouchableHighlight component to enable press events, it can be styled with background color and border to look like a button. With NativeBase, you get such buttons out-of-the-box along with contextual colors for different use-case scenarios.
Similarly, NativeBase also has easy-to-customize List components with thumbnail, icons and muted text. You can check it in detail here: http://nativebase.io/docs/v0.2.1/components