The Domain Name System is used to determine what IP address is associated with a given domain name. So, to reverse-resolve a known IP address is to look up what the associated domain name is belonging to that IP address.

RDNS lookups for IPv4 addresses use a reverse IN-ADDR entry in the special domain in-addr.arpa. An IPv4 address is represented in the in-addr.arpa domain by a sequence of bytes in reverse order, represented as decimal numbers, separated by dots with the suffix .in-addr.arpa. For example, the reverse lookup domain name corresponding to the IPv4 address 69.65.121.101 is 101.121.65.69.in-addr.arpa.