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 collection of all records for the domain address, so when you open a URL inside an Internet browser, your personal computer asks the DNS servers around the globe 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 web site content is requested from the proper location, a mail relay server discovers which server takes care of the emails for the domain address (MX record) to ensure a message can be sent to the needed mailbox, and so on. Any modification of these sub-records is performed using the company whose name servers are employed, permitting you to keep the website hosting and change only your email provider for example. Every single domain name has at least 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.