Use this property to enable communication between a Web page hosted by the WebBrowser control and the application that contains the WebBrowser control. This property lets you integrate dynamic HTML (DHTML) code with your client application code. The object specified for this property is available to Web page script as the window.external object, which is a built-in DOM object provided for host access.
You can set this property to any COM-visible object for which you want its public properties and methods available to scripting code. You can make a class COM-visible by marking it with the ComVisibleAttribute.