DNS basically is the translation of "mydomain.com" to an IP address such as 192.168.0.1. Reverse DNS is going from an IP address to the domain name. Reverse DNS is mainly used for mail programs to trace back to a domain. You can test reverse DNS settings by going to
www.dnsstuff.com and using the box at the top of the screen.
To set up Reverse DNS you have a line in your server and your upstream provider makes an entry in their system configuration. Ask your upstream provider about it and they can provide more details. It only takes a few minutes for them to setup.