These are files added to the server that allow you to use the advaced features, also called components, of Front Page. There is a lot of controversy on the use of these extensions. On our hosting packages, these extensions are turned off by defaut but you have the option to turn them on. If you don't know exactly what you are doing, leave them off. If you turn them on, you must always use Front Page to publish (upload) your website otherwise your site may not work properly.

Allows the features in Microsoft FrontPage to work correctly with the server

"The FrontPage Server extensions are a set of programs that sit on the Web server and extend FrontPage functionality." Read More at Auxservices

Server side programs that enable Front Page users to utilize the progam's special components.