The DNS service is displayed when you choose DNS in the Service pop-up menu in the Test Info pane.
This service attempts to resolve a domain name, i.e. that this computer knows where that domain is.
Unlike the Specific DNS service, this one only checks the default DNS server for your Mac. If you want to use another server, use the Specific DNS one instead.
Domain Name: Enter a domain name to look up. This service will very quickly check that it can resolve the entered domain name to an IP address. This doesn't prove that the server at that address is actually available, but simply that it knows where the server is (or is advertised by the domain name server to be).