Where does this `webpack://` come from for `webpack-dev-middleware`?

I'm using http://ift.tt/1NP2o79.

When I view the source code in Chrome, I saw a source is called webpack:// (see the picture below)

However, when I access the URL such as webpack:///src/App.js?14b1. I find the file is not accessible.

Does anyone have ideas about where does this webpack:// sources come from?