Reverse DNS (or sometimes called as PTR record) is a DNS record that helps to determine the domain name that is associated with a given IP address using the Domain Name System (DNS) of the Internet.

PTR record is the opposite of an A record. While the A record points a domain name to an IP address, the PTR record resolves the IP address to a domain/hostname. PTR records are used for the reverse DNS (Domain Name System) lookup. Using the IP address you can get the associated domain/hostname as a result of the lookup. An A record should exist for every PTR record.
Using a reverse DNS setup for a mail server is a good solution. Some external mail exchange servers make reverse DNS lookups before accepting messages originating from your mail server.

More information is available via the link below:

Wikipedia on PTR Record / Reverse DNS