The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Basically, the zone is the group of all records for the domain name, so when you open a URL within an Internet browser, your personal computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain name should be retrieved. This way a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the site content is requested from the proper location, a mail relay server detects which server manages the emails for the domain name (MX record) to ensure that a message can be sent to the needed mailbox, and so forth. Any change of these sub-records is performed with the help of the company whose name servers are employed, so that you can keep the web hosting and switch only your email provider for instance. Every single domain name has no less than 2 NS records - primary and secondary, that start with a prefix like NS or DNS.