The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Simply, the zone is the range of all records for the domain name, so when you open a URL in a browser, your personal computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain address ought to be retrieved. This way a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP and the website content is requested from the proper location, a mail relay server finds out which server handles the emails for the domain name (MX record) so that a message can be forwarded to the appropriate mailbox, etc. Any change of these sub-records is performed using the company whose name servers are used, so you can keep the website hosting and change only your email provider for example. Each and every domain has no less than 2 NS records - primary and secondary, that start with a prefix like NS or DNS.