DNS stands for Domain Naming Server. DNS acts as a translator between domain names and IP addresses. As humans remember names, the computer understands only numbers. Generally, we assign names to websites and computers like Gmail.com, Hotmail, etc. When we type such names the DNS translates it into numbers and executes our requests.
Translating the names into numbers or IP address is named as a Forward lookup.
Translating the IP address to names is named as a Reverse lookup.