The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Simply, the zone is the selection of all records for the domain name, so when you open a URL within an Internet browser, your PC asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain address ought to be retrieved. With this a web browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP address and the web site content is requested from the correct location, a mail relay server detects which server manages the emails for the domain (MX record) to ensure a message can be sent to the appropriate mailbox, etc. Any change of these sub-records is conducted using the company whose name servers are employed, enabling you to keep the web hosting and change only your email provider for instance. Every Internet domain has at least 2 NS records - primary and secondary, that start with a prefix like NS or DNS.