THe Domain Name Server (DNS) is where your PC, NAS etc go to lookup IP addresses form a hostname. If for example youe type www.google.com then the DNS looks up the IP address of that domain.
If you do not run your own DNS server, then enter the DNS server IP addresses provided by your ISP, the same ones that you use for your computer. If your somputer uses dynamic addressing (DHCP) then it may get the DNS server IP addresses automatically when you connect to the internet. You can display your DNS server addresses (and other information) by starting a command window and typing "ipconfig /all".