A mirror, in computing, is a server dedicated for having duplicate content of another server. This is done for several reasons:
to allow faster downloads for users in one area: for example, a US server could be mirrored in Japan, allowing Japanese Internet users to download content faster from the Japanese server than the American
to balance load - if one server is extremely popular a mirror may help relieve this load: for example if a Linux distribution is released as an ISO image onto the distribution producer's own server, this server may be overloaded with demand and so mirrors may help reduce the load off this main server
to keep historic content: economic reasons may prevent the maintainers of a server from keeping older and unsupported content for users who still may desire them - a mirror may be made to keep this content from disappearing
