DNS stands for Domain Name Server. It is the IP address of the computer that turns names like 'www.joeserver.com' into an IP address like '192.168.1.1'
Set the DNS that Valid Sender, Bouncer, and EMail Stamps will use to deliver e-mail. Usually the default automatic detection works fine. If not, you have the option to set your DNS server. You can use the Test DNS button to test the default automatic detection or your own custom DNS setting.