PropertyFor is a method in EPiServer CMS's API. It's a static method of the PropertyExtensions class in the EPiServer.Web.Mvc.Html namespace and it's an extension method for HtmlHelper objects in ASP.NET MVC.
When executed in a request that is outside of EPiServer's edit mode it acts as a thin wrapper for ASP.NET MVC's DisplayFor method while in EPiServer's edit mode it extends the output from DisplayFor to add HTML elements and other markup that is needed for EPiServer CMS to recognize properties that can be edited.
I've found that a deep understanding of how the PropertyFor method works is key to building all but the most trivial of sites using EPiServer CMS and ASP.NET MVC. Under this tag I collect articles and notes about how the PropertyFor method works and real life examples of using it.