Metro team keeps improving its documentation, now you can find some really good explanations at https://facebook.github.io/metro/docs/concepts (link updated):
your code and its dependencies.
So yes, it is a sort of Webpack, but for React Native apps 🙂
But where is the bundle file located?
Once the bundler is started, you can check its contents at http://localhost:8081/index.bundle?platform=ios&dev=true&minify=false
(like webpack, it is served from memory, so it is not being written on your project’s folder).
What is the use of that bundle file ?