and for the average home web-designer it's something that is best forgotten about, and to be honest I would advise my corporate clients to stay clear of it as well - it's seldom necessary, as Taran says.
The only 'mirror' that should really interest most people is the back up copy of their site which resides on their hard drive (and this is often what people mean when they refer to a site mirror). This copy is the one you should be working from - not the server copy. I've lost count of the number of people who have asked me to help when they have indulged in long and complex editing sessions 'live' on the server copy, have forgotten to save the site back to their hard drive, and have subsequently used the hard drive copy to upload a minor alteration to the server.......overwriting all their previous hard work in the process. There's nothing that can be done when that happens, except deliver my (unwelcome) little lecture on working practices.