What is WebResource.axd?

WebResource.axd provides access to embedded resources within a project. It's a handler that enables control and page developers to download resources that are embedded in an assembly to the end user.

You include WebResources in your AssemblyInfo:

[assembly: System.Web.UI.WebResource("Project.Styles.Main.css", "text/css")]

Then you can get an include path for your Page using the following code:

string includeLocation = Page.ClientScript.GetWebResourceUrl(this.GetType(),    
    "Project.Styles.Main.css");

Then to add the above file (which is a CSS file in this case):

LiteralControl include = new LiteralControl(
    String.Format(includeTemplate, includeLocation));

((System.Web.UI.HtmlControls.HtmlHead)Page.Header).Controls.Add(include);

Then you'll end up seeing references within your page source such as the following:

/WebResource.axd?d=PhPk80h_UWEcbheb-NHNP5WshV_47UOpWqAOl1_li
    UFfN4cNofL74cFlQ1fvpFSf0&t=632573240669964903

Its for embedded resources.

Working with Web Resources in ASP.NET 2.0

It's a generated HttpHandler file. Most likely you have some embedded resource in your ASP.NET application, or one of the third party components you use. To transport these in a transparent way ASP.NET makes use of this approach where the correct resource can be identified depending on the passed in querystring. For example:

WebResource.axd?d=PhPk80h_UWEcbheb-NHNP5WshV_47UOpWqAOl1_liUFfN4cNofL74cFlQ1fvpFSf0&t=632573240669964903

Taken from Using WebResource.axd for embedded resources.