The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain, so when you open a URL inside a browser, your computer asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain name should be retrieved. 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 site content is requested from the proper location, a mail relay server finds out which server deals with the emails for the domain (MX record) so that a message can be forwarded to the appropriate mailbox, and so forth. Any modification of these sub-records is done with the help of the company whose name servers are employed, allowing you to keep the website hosting and change only your email provider for example. Each and every domain name has a minimum of 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.