The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain, so when you open a URL within an Internet browser, your PC 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. That 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 and the site content is requested from the proper location, a mail relay server finds out which server handles the e-mails for the domain (MX record) to ensure that a message can be delivered to the right mailbox, and so on. Any modification of these sub-records is performed through the company whose name servers are employed, enabling you to keep the website hosting and switch only your email provider for instance. Each and every domain name has a minimum of 2 NS records - primary and secondary, that start with a prefix like NS or DNS.