The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Simply, the zone is the group of all records for the domain name, so when you open a URL in an Internet browser, your PC asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain name 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 required from the correct location, a mail relay server discovers which server handles the e-mails for the domain (MX record) so a message can be delivered to the appropriate mailbox, and so forth. Any change of these sub-records is performed with the help of the company whose name servers are used, permitting 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, which start with a prefix like NS or DNS.