DNS stands for Domain Name System. It is a system that takes the name of a website you ask for, say opennicproject.org and translates it into the number that the server hosting the website is at 126.96.36.199, a bit like a house has a number.
DNS is used because remembering the number of a server is not easy, especially if you want to remember lots! A name is much easier to remember.
Please watch this video for a better view of how DNS works.