What DNS Service mean?

All the computers in the internet should have a unique address called IP address(for eg: 32 bit address in the form of xx.xx.xx.xx). But, it is very hard for we humans to remember all those numbers. So, we generally refer to them as their English names (domain names). So, whenever we refer to any domain name, it has to be converted into IP address. This operation of resolving a domain name into corresponding IP address is done by a DNS server.

DNS stands for Domain Name Service. As an analogy, you can take an example of the telephone directory where you look for the phone numbers for a person's name.