The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Simply, the zone is the selection of all records for the domain address, so when you open a URL in a browser, your PC asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain name should be retrieved. With this 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 and the web site content is requested from the proper location, a mail relay server detects which server handles the emails for the domain address (MX record) so that a message can be forwarded to the needed mailbox, and so forth. Any modification of these sub-records is conducted with the help of the company whose name servers are used, so that you can keep the website hosting and switch only your email provider for instance. Each domain name has a minimum of two NS records - primary and secondary, which start with a prefix such as NS or DNS.