What's the "een" in Halloween?
An archive site or website which keeps a copy of some or all files at another site so as to make them more quickly available and to reduce the load on the source site.
It is generally best to use the mirror that is physically closest to you as this will usually give the fastest download.
Such mirroring is usually done for specific whole directories or files on a specific remote server as opposed to a cache or proxy server which keeps copies of everything that is requested via it.
For example, src.doc.ic.ac.uk is the main UK mirror for the GNU archive at gnu.org.