What is the _references.js used for?

In VS 11, Visual Studio will give you intellisense from all files that have references in the “_references.js” file.

For More Info

MSDN documentation: JavaScript IntelliSense

In brief, _references.js stores the list of JS files paths, for which you want the Visual Studio to gather and build the intelisense (aka "code complete"). The VS project adds there some common JS libraries like jQuery to build the intellisense for it. You can add a reference to your custom JS file in form like this:

/// <reference path="MyScriptFile.js" />

From Visual Studio 2017 forward, this file is no longer used.