The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Simply, the zone is the collection of all records for the domain address, so when you open a URL in a web browser, your personal computer asks the DNS servers globally where the domain name is hosted and from which servers the DNS records for the domain should be retrieved. In this way a browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP address and the site content is requested from the proper location, a mail relay server discovers which server handles the e-mails for the domain (MX record) to ensure a message can be sent to the needed mailbox, etc. Any change of these sub-records is conducted through the company whose name servers are used, so that you can keep the web hosting and change only your email provider for instance. Each and every domain address has at least 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.