The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Basically, the zone is the group of all records for the domain, so when you open a URL inside a browser, your personal computer asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain should be retrieved. 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 Internet protocol address and the site content is requested from the proper location, a mail relay server discovers which server handles the emails for the domain name (MX record) so a message can be delivered to the needed mailbox, and so forth. Any modification of these sub-records is performed with the help of the company whose name servers are employed, enabling you to keep the website hosting and switch only your email provider for instance. Every single domain name has no less than 2 NS records - primary and secondary, which start with a prefix like NS or DNS.
