The NS (Name Server) records of a domain name 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 personal computer asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain name ought to 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 website content is requested from the right location, a mail relay server finds out which server deals with the e-mails for the domain (MX record) so a message can be sent to the correct mailbox, and so forth. Any change of these sub-records is conducted using the company whose name servers are used, allowing you to keep the website hosting and change only your email provider for instance. Each Internet domain has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.