My own knowledge of the subject is not very deep. Yes the mirror is something that copies your website content so it would be accessible when your main server is offline.
The simplest approach is to be able to access the copied content by a different url. I have no extensive knowledge how to implement dynamic mirroring so it would kick in automatically, when the main server goes down. I guess it would require playing with dns records, so that the mirroring server would constantly check if the main server is online, and if not, it would register dns records to point to itself instead.
Some information i found:
http://en.wikipedia.org/wiki/Mirror_(computing)
http://wenderhost.com/2007/06/11/mirror-a-dynamic-website-on-a-cpanelwhm-server/
