Complex sentence: Typically, the DNS is used to determine what IP address is associated with a given hostname; so, to reverse resolve a known IP address is to lookup what the associated hostname is belonging to that IP address.
Split into simpler sentences: Typically, the Domain Name System is used to determine what IP address is associated with a given host name. So, to reverse-resolve a known IP address is to look up what the associated host name is belonging to that IP address.