The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain name, so when you open a URL in a web browser, your PC asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain must 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 IP address and the site content is requested from the proper location, a mail relay server finds out which server takes care of the e-mails for the domain address (MX record) to ensure a message can be sent to the correct mailbox, etc. Any change of these sub-records is done using the company whose name servers are used, allowing you to keep the website hosting and switch only your email provider for example. Every single Internet domain has a minimum of two NS records - primary and secondary, which start with a prefix such as NS or DNS.