The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain name, so when you open a URL in a browser, your PC asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain ought to be retrieved. With this a browser finds out what the A or AAAA record of the domain name 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 emails for the domain (MX record) so a message can be forwarded to the correct mailbox, etc. Any change of these sub-records is done with the help of the company whose name servers are used, enabling you to keep the website hosting and change only your email provider for instance. Every single Internet domain has a minimum of 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.