The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Simply, the zone is the group of all records for the domain, so when you open a URL within a browser, your computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain must be retrieved. That way a 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 web site content is requested from the proper location, a mail relay server discovers which server handles the e-mails for the domain name (MX record) so a message can be sent to the correct mailbox, and so forth. Any modification of these sub-records is performed using the company whose name servers are employed, enabling you to keep the web hosting and switch only your email provider for instance. Every single domain has at least two NS records - primary and secondary, that start with a prefix like NS or DNS.