Reverse DNS is IP address to domain name mapping - the opposite of forward (normal) DNS which maps domain names to IP addresses.
You may need to register a reverse domain, for example, if you need to access your server to a server configured in such a way that it checks for records of your host in the forward and reverse zones. If there is no entry in one of them, access to the service from your host can be closed.
Regarding entries in the reverse zone, you should contact the provider who allocated IP addresses from your network. For example, if you are assigned an IP address of 194.85.61.42 on the network 194.85.61.0/24, then a record of your host (for example, www.mydomainname.ru) will be made in the reverse zone 61.85.194.in-addr.arpa. The entry will look like:
42 PTR www.mydomainname.ru.
If you registered a block of ip-addresses of the form: 100.101.102.0 - 100.101.103.255, then you should place the reverse zones:
102.101.100.in-addr.arpa
103.101.100.in-addr.arpa
on DNS servers, make the necessary PTR records in them, and then register the reverse domains in the RIPE database. This is usually done by the network administrator.
If you do not have your own DNS-servers, on which you could place return zones, you can order their support in RU-CENTER.