The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain address, so when you open a URL inside an Internet browser, your computer asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain name ought to be retrieved. In this way a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP address and the web site content is requested from the proper location, a mail relay server detects which server takes care of the emails for the domain (MX record) so a message can be sent to the needed mailbox, etc. Any change of these sub-records is performed using the company whose name servers are used, so that you can keep the website hosting and switch only your email provider for instance. Every domain name has no less than 2 NS records - primary and secondary, that start with a prefix like NS or DNS.