A mirror site is a site that also has the same download file. Many places do this to save load on their servers because their servers cannot cope with the traffic & bandwidth needed for constant downloading of their software. Thus, sites around the world will share the same file that can be downloaded from. It's suggested to choose the nearest one to where you live so it will be faster, however it's not so important. I downloaded from France which is on opposite side of world from me. lol