The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the range of all records for the domain address, so when you open a URL within a browser, your PC asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain should be retrieved. In this way a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP and the site content is required from the proper location, a mail relay server discovers which server manages the e-mails for the domain (MX record) so a message can be sent to the appropriate mailbox, and so on. Any change of these sub-records is performed through the company whose name servers are used, allowing you to keep the web hosting and change only your email provider for instance. Each and every domain address has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.