What is a Web.config file? How do you edit a Web.config file? Here are some Web.config Basics. A Web.config file is used to configure various settings for a website running on Microsoft IIS. It is structured using XML in a file that is separate then your website/application’s code. You can have a single Web.config, or several spread throughout folders on your site – they can assign different permissions, references, application variables, setup caching, and more!