The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Simply, the zone is the selection of all records for the domain address, so when you open a URL inside 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 name ought to be retrieved. In this way a browser finds out what the A or AAAA record of the domain address 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 e-mails for the domain address (MX record) to ensure that a message can be forwarded to the correct mailbox, and so forth. Any change of these sub-records is done with the help of the company whose name servers are used, allowing you to keep the web hosting and switch only your email provider for instance. Every domain address has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.