What are mirrors?

I'm sorry for asking, I've done a search but not come up with anything. Please could you explain what 'mirrors' are when downloading files.Thank you.

another server with exactly the same content.

Mirrors are put up just in case you are unable to access the file on the server, therefore, they are used as alternate files hosted on different servers just in case.The alternate servers are free servers... like rapidshare.Sooo I hoped that clarified that topic up a bit!

actually there are two romulation owned mirrors, its not just in case you are unable to access one, its to balance load as well

Oh I see, thank you so clearing that up .