A website mirror is an alternative address (URL) where you can find this website.
Sometimes the main site address is unavailable for some reason. For example, some countries have laws prohibiting online gambling. They block access to gambling sites.
For their part, bookmakers create alternative links(mirror sites) to help their bettors
Let's take a look at http://www.allsport365.com which is a mirror http://www.bet365.com . When you use allsport365, you will get to the same place as if you were using bet365.com . It works like another door to the same room.
Another case is when a bookmaker may not be available from anywhere in the country where the bookmaker does not have a national betting license.
In this case, you should use a mirror or try to find a clone who has a national betting license in your country.