The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain, so when you open a URL within a web browser, your computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain address should be retrieved. That way a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the website content is requested from the right location, a mail relay server detects which server manages the emails for the domain (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 employed, allowing you to keep the website hosting and switch only your email provider for instance. Each and every domain has a minimum of 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.