The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Simply, the zone is the range of all records for the domain address, so when you open a URL in a web browser, your PC asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain name must 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 website content is requested from the correct location, a mail relay server discovers which server takes care of the emails for the domain name (MX record) to ensure that a message can be forwarded to the needed mailbox, etc. Any change 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 a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.