Beyond all technicalities, it is a very simple and attribute-based way of exposing any arbitrary server-side function to LINQ. It goes beyond what LINQ to SQL does with SqlMethods and it leverages our metadata system so that you don't have to specify the full mapping of parameters in the attribute.
The post itself may be a little boring ;), but the scenarios it enables are quite impressive.
Read more here.
Update: I remembered today that Kati Dimitrova and Sheetal Gupta also contributed to the document.