The rDNS stands for Reverse Domain Name System. If you are familiar with the DNS, you know that the most common case is a forward DNS lookup. A typical request is when a specific domain name is written, and the DNS must find the corresponding IP address.
The rDNS is exactly the opposite operation. What is requested is the corresponding domain of a specific IP address.
This operation is also known as reverse DNS resolution or reverse DNS lookup (rDNS) in computer networks. Frequently, rDNS is considered a premium feature or service supplied by paid managed DNS plans.