Quick answer: Reverse DNS uses a PTR record to map an IP address to a hostname, if one has been published.
Forward vs reverse DNS
Forward DNS starts with a name and returns an address. Reverse DNS starts with an address and asks for its PTR hostname. The two are separate records and can exist independently.
Why reverse DNS is useful
Network operators and mail systems often inspect PTR records as one clue about how an IP is configured. A PTR name is not proof of a user’s identity or physical location.
Try DNS Lookup and then read Reverse DNS and PTR Records for the deeper explanation.
Want to see this in practice? Check your own connection's IP, location and ISP.
Check my IP address