The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Basically, the zone is the collection of all records for the domain name, so when you open a URL inside a web 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. In this 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 address and the web site content is requested from the correct location, a mail relay server finds out which server handles the emails for the domain (MX record) so that a message can be forwarded to the correct mailbox, and so on. Any change of these sub-records is done through the company whose name servers are used, permitting you to keep the website hosting and switch only your email provider for instance. Each and every domain name has no less than 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.