The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain, so when you open a URL in a browser, your personal computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain address ought to be retrieved. That 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 detects which server deals with the emails for the domain (MX record) so a message can be forwarded to the correct mailbox, etc. Any change of these sub-records is performed using the company whose name servers are employed, so that you can keep the web hosting and change only your email provider for instance. Each domain name has a minimum of 2 NS records - primary and secondary, which start with a prefix like NS or DNS.