Reverse DNS is the system by which you can give a name to each of your IP addresses. Normally DNS gives an IP address to a name, but reverse DNS is the other way around.
Sometimes it is necessary to have a name associated with an IP address, and then have that IP address correctly associated with a name. This is because some systems and tools on the internet will check the address (e.g. some ftp servers and some email servers). You should only set reverse DNS to domain names you control.
The way this works is that the IP address is turned in to a domain, e.g. 126.96.36.199 would be turned in to 188.8.131.52.in-addr.arpa. Any machine wanting to know the name for 184.108.40.206 would look up the PTR DNS record on 220.127.116.11.in-addr.arpa.
We are delegated the address blocks we control by RIPE, e.g. we manage 0.169.217.in-addr.arpa. (and many others). If you had, for example, a block of addresses from 18.104.22.168 to 22.214.171.124 then you might want to give names to those addresses.
This may sound rather complicated, but we have several ways to manage your reverse DNS. Please see the Knowledge Base articles below for further technical information.