DNS refers to Domain Name Server.
When you want to access a website, you go ahead to the web browser and then enter web address.
Web address can be website real Address (IP Address) like: 220.127.116.11, or Web Address like: moradhamdy.com.
IP Address is hard to remember, but Web Address is much easier.
Real web address are the IP Address form. So The Domain Name Servers is invented to match this ip with the web address.