What is a Reverse lookup Zone into DNS Server?

Normally  forward lookup is used where in the name of a domain is resolved to IP address as IP's are hard to remember and we use names and DNS resolved it to IP address.

The vice versa is reverse look up. SO simply put when an IP address is used to find the name of a domain it is reverse look up.Hope this helps

Forward lookup zone helps changing Domain name to Ip address details

Reverse lookup zone helps changing IP address to Domain name details(Just the reverse of Forward lookup zone)

Reverse lookup zone resolving hostname to address.  Detailed description below

DNS also provides a reverse lookup process, in which clients use a known IP address and look up a computer name based on its address. A reverse lookup takes the form of a question, such as "Can you tell me the DNS name of the computer that uses the IP address192.168.1.20?"

DNS was not originally designed to support this type of query. One problem in supporting the reverse query process is the difference in how the DNS namespace organizes and indexes names and how IP addresses are assigned. If the only method to answer the previous question is to search in all domains in the DNS namespace, a reverse query would take too long and require too much processing to be useful.

To solve this problem, a special domain, the in-addr.arpa domain, was defined in the DNS standards and reserved in the Internet DNS namespace to provide a practical and reliable way to perform reverse queries. To create the reverse namespace, subdomains within the in-addr.arpa domain are formed, using the reverse ordering of the numbers in the dotted-decimal notation of IP addresses.

This reversed ordering of the domains for each octet value is necessary because, unlike DNS names, when IP addresses are read from left to right, they are interpreted in the opposite manner. When an IP address is read from left to right, it is viewed from its most generalized information (an IP network address) in the first part of the address to the more specific information (an IP host address) that is contained in the last octets.

For this reason, the order of IP address octets must be reversed when the in-addr.arpa domain tree is built. The IP addresses of the DNS in-addr.arpa tree can be delegated to organizations as they are assigned a specific or limited set of IP addresses within the Internet-defined address classes.