Mirror of a website is an alternative address(URL) where you can find this website. Sometimes main address of the site is unreachable for some reasons. For example some countries has laws against online gambling. They block the access to gambling sites. From their side bookmakers create alternative links for their customers.
Let's glance at http://www.allsport365.com which is a mirror of http://www.bet365.com. When you use allsport365, you will come to the same place as if you have used bet365.com . It works like another door to the same room.
Another case when a bookmaker could be unreachable from somewhere is the bookmaker has no national betting license.
In this case you should use a mirror or try to find a clone
who has the national betting license of your country.