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 an Internet browser, your PC asks the DNS servers around the globe 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 address is so that the latter is mapped to an IP address and the site content is requested from the right location, a mail relay server detects which server handles the emails for the domain address (MX record) so a message can be sent to the correct mailbox, and so forth. Any modification of these sub-records is conducted through the company whose name servers are employed, permitting you to keep the website hosting and change only your email provider for instance. Each and every Internet domain has no less than 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.