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 selection of all records for the domain address, so when you open a URL inside an Internet browser, your laptop or computer asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain should 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 IP address and the web site content is required from the proper location, a mail relay server discovers which server handles the emails for the domain (MX record) so that a message can be forwarded to the needed mailbox, and so forth. Any change of these sub-records is done using the company whose name servers are used, enabling you to keep the website hosting and change only your email provider for instance. Each and every domain name has a minimum of 2 NS records - primary and secondary, that start with a prefix like NS or DNS.