The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. In simple terms, the zone is the range of all records for the domain, so when you open a URL within a browser, your PC 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 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 proper location, a mail relay server finds out which server handles the e-mails for the domain address (MX record) so a message can be sent to the right mailbox, etc. Any modification of these sub-records is done through the company whose name servers are used, allowing you to keep the web hosting and switch only your email provider for example. Each and every domain has no less than 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.