The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Basically, the zone is the group of all records for the domain address, so when you open a URL in a web browser, your computer asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain must be retrieved. That 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 and the site content is requested from the proper location, a mail relay server discovers which server takes care of the emails for the domain name (MX record) so a message can be forwarded to the needed mailbox, and so on. Any modification of these sub-records is conducted with the help of the company whose name servers are employed, so you're able to keep the web hosting and switch only your email provider for instance. Every domain address has a minimum of 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.