The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Simply, the zone is the collection of all records for the domain address, so when you open a URL in an Internet browser, your laptop or computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain name ought to be retrieved. That 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 web site content is requested from the right location, a mail relay server finds out which server deals with the emails for the domain (MX record) so a message can be delivered to the appropriate mailbox, etc. Any modification of these sub-records is conducted with the help of the company whose name servers are used, enabling you to keep the web hosting and change only your email provider for instance. Each domain name has at least 2 NS records - primary and secondary, that start with a prefix like NS or DNS.