A 'mirror' is when the exact same file is hosted at a different location. This used to be a different phycal location or machine, but I guess it could be just a different logical location with virtualization these days.
When you download something from sourceforge or a linux ISO, you usually choose the mirror that's closest to you for the best physical speed.
Some sites may consider mirroring a file on both rapidshare and megaupload, for example.