App_Start is just another folder that groups together ASP.NET MVC configuration, which in previous versions of ASP.NET MVC was done in Global.asax.
ASP.NET MVC introduces more and more configuration elements, and this folder is ideal to place this configuration. For example, MVC 5’s new auth. configuration, such as for third-party login providers, are also placed within this folder (in Startup.Auth.cs).
App_Start is not a ASP.NET special folder recognized by ASP.NET/IIS. You can rename the folder if you want. The name is just a convention, like App_GlobalResouces etc.
Update:
Below are some information and reference points for each file. The use of these files are pretty straightforward. I have included few online references that might help your to understand more.
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AuthConfig— registers external authentication providers.
See ASP.NET MVC external authentication providers for more information. -
BundleConfig— registers your CSS and JS so they can be bundled and minified. See also ASP.NET MVC: Guidance: Bundling and Minification. -
WebApiConfig— only applicable if you are using Web API. It can be used to configure Web API-specific routes, any Web API settings and Web API services. See also configuring ASP.NET MVC Web API 2 -
FilterConfig— registered global filters. These filters are applied to all actions and controllers. See also ASP.NET MVC 3: Global action filters -
RouteConfig— you already found information.