The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Simply, the zone is the range of all records for the domain name, so when you open a URL in an Internet browser, your personal computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain ought to be retrieved. This way a 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 required from the proper location, a mail relay server detects which server takes care of the e-mails for the domain address (MX record) so that a message can be sent to the right mailbox, etc. Any modification of these sub-records is conducted with the help of the company whose name servers are employed, so you're able to keep the web hosting and change only your email provider for instance. Each and every Internet domain has a minimum of 2 NS records - primary and secondary, which start with a prefix like NS or DNS.