The backbone of this test is using DNSCheck
DNSCheck is a program that was designed to help people check, measure and hopefully also understand the workings of the Domain Name System, DNS. DNSCheck was created for .SE - The Internet Infrastructure Foundation. The source code for DNSCheck can be viewed on Github
When a domain (aka zone) is submitted to DNSCheck it will investigate the domain's general health by traversing the DNS from root (.) to the TLD (Top Level Domain, like .SE) to eventually the name server(s) that holds the information about the specified domain (like iis.se). Some other sanity checks, for example measuring host connectivity, validity of IP-addresses and control of DNSSEC signatures will also be performed.
About the domain name system, DNS
The domain name system (DNS in short) is what could be called the "phone book" of the Internet. It keeps track of the mapping of, for example, a human-readable website name (like www.iis.se) to the slightly more arcane form of an IP-address that the computer needs to initiate communication (in this case 18.104.22.168).
Besides browsing the Internet with your web browser using website names instead of IP-addresses the DNS also makes sure your emails find their way. In short, a stable DNS is vital for most companies to maintain a working and efficient operation.