The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain, so when you open a URL in a web browser, your computer asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain address must be retrieved. With this a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP address and the web site content is required from the proper location, a mail relay server finds out which server handles the e-mails for the domain (MX record) so that a message can be forwarded to the appropriate mailbox, and so on. Any change of these sub-records is done using the company whose name servers are used, enabling you to keep the website hosting and change only your email provider for example. Each and every domain has a minimum of 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.