The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain address, so when you open a URL within an Internet browser, your personal computer asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain address must be retrieved. That 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 correct location, a mail relay server discovers which server deals with the e-mails for the domain name (MX record) so a message can be forwarded to the needed mailbox, etc. Any modification of these sub-records is conducted with the help of the company whose name servers are used, so that you can keep the website hosting and change only your email provider for instance. Each and every Internet domain has a minimum of 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.
