The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain name, so when you open a URL in a web browser, your PC asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain name should be retrieved. In 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 web site content is requested from the right location, a mail relay server discovers which server takes care of the e-mails for the domain address (MX record) so that a message can be sent to the right mailbox, and so on. Any modification of these sub-records is done using the company whose name servers are employed, allowing you to keep the website hosting and change only your email provider for instance. Every Internet domain has no less than 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.