The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Essentially, the zone is the range of all records for the domain name, so when you open a URL in a browser, your laptop or computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain should 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 website content is requested from the proper location, a mail relay server finds out which server manages the emails for the domain name (MX record) so a message can be forwarded to the appropriate mailbox, etc. Any modification of these sub-records is conducted using the company whose name servers are used, so that you can keep the website hosting and change only your email provider for instance. Every domain has at least 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.