DNS stands for Domain Name Server, and is a service provided to help translate IP addresses into readable addresses. Eg. 204.127.202.4 = ns5.attbi.com
People seem to be able to interact and remember names more easily than a bunch of numbers, so the DNS function helps us do that. If you start a Command Prompt on your computer, type in ipconfig /all, the address you mentioned should be listed next to the entry: DNS Servers . . . . . . . . . . . : 204.127.202.4
The DNS function on your computer communicates with this server using UDP port 53, so it’s safe to allow and “remember this” for later use.