The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. In simple terms, the zone is the selection of all records for the domain name, so when you open a URL within a browser, your personal computer asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain address should be retrieved. That way a web browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the site content is requested from the correct location, a mail relay server detects which server handles the emails for the domain (MX record) so that a message can be forwarded to the needed mailbox, etc. Any change of these sub-records is done using the company whose name servers are employed, enabling you to keep the website hosting and switch only your email provider for example. Every domain has at least 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.