From Unofficial BOINC Wiki
A Server that performs the task of providing files requested using the FTP Protocol and has the same contents as another FTP Server so that the speed of response can be increased along with the total number of users serviced at the same time. The effect of the mirror is to increase the ability to service the requests. The mirror may be accessed through a separate address or in some cases by redirection of the requests. There can be more than two computers providing mirroring. In some cases, mirroring is provided on a geographically dispersed basis. Users in Europe might be getting files from a server located in France, while the users on the east coast of the United States are serviced by a second, and the west coast users are serviced by yet a third mirror.