They are trouble. Unfortunately a lot of people want them though.
They are a set of server-side scripts/programs designed to work with MS Frontpage. They provide functionality such as form handling, auto-includes, hit counters and a bunch of other stuff.
Click here for info about installing them on your server.
FP extensions are prone to inexplicable failure. It probably makes sense to say they are available but I wouldn't encourage people to use them. In fact they are so troublesome that we offer them only at the customer's risk - we make no guarantee about their reliability and we charge for any FP-related support. A lot of hosts simply refuse to offer them (which upsets the folks at microft.public.frontpage no end!)