Would someone mind explaining Reverse DNS with Virtual Hosts please?

Hi guys,

I'm not 100% sure this is best place for such a question, but I cannot get my head around reverse DNS settings with virtual hosts.

I have one IP address, Apache2 with a number of virtual hosts, using iRedMail for serving email, and running 12.04 Ubuntu on a VPS.

All of my host records are configured over with Namecheap and everything is working fine other than getting warnings on intodns.com about not having reverse DNS correctly set up in my records.

An "Explain like I'm 5" would be even better!
Thank you in advance.

EDIT: Thank you all very much for your information and answers, I now have it set up correctly. I really appreciate all of your helpful and supportive answers.