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 name, so when you open a URL within an Internet browser, your computer asks the DNS servers world-wide where the domain address is hosted and from which servers the DNS records for the domain name 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 and the web site content is required from the correct location, a mail relay server detects which server takes care of the emails for the domain address (MX record) so a message can be sent to the needed mailbox, and so on. Any modification of these sub-records is performed using the company whose name servers are used, so you can keep the website hosting and switch only your email provider for instance. Each and every domain has no less than 2 NS records - primary and secondary, that start with a prefix like NS or DNS.