A Reverse IP Lookup is a way of identifying hostnames which have DNS (A) records associated with an IP, This hostname could be an underlining provider or ISP. It is worth noting many hosting services allow you to assign a reverse IP hostname which is useful for mail servers.
What is a Reverse IP Lookup?
A Reverse IP lookup (or reverse DNS lookup) is the process of querying the Domain Name System (DNS) to determine the domain name associated with an IP address, which is the reverse of the usual "forward" DNS lookup of an IP address from a domain name.
What does it tell you?
It queries Pointer (PTR) records to find the hostname. This tool also cross-references the IP address against ASN databases and Geolocation services to provide you with the exact physical location, Internet Service Provider (ISP), and geographical continent of the requested IP address.