The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the range of all records for the domain, so when you open a URL in an Internet browser, your computer asks the DNS servers around the globe where the domain address 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 is so that the latter is mapped to an Internet protocol address and the web site content is required from the proper 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 delivered to the correct mailbox, and so forth. Any modification of these sub-records is conducted through the company whose name servers are employed, so that you can keep the web hosting and change only your email provider for instance. Every domain has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.