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 in a browser, your PC asks the DNS servers worldwide where the domain address is hosted and from which servers the DNS records for the domain name must be retrieved. With this a browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP address and the web site content is requested from the correct location, a mail relay server finds out which server deals with the emails for the domain (MX record) to ensure that a message can be forwarded to the appropriate mailbox, etc. Any change of these sub-records is done with the help of the company whose name servers are used, so you can keep the web hosting and change only your email provider for example. Each and every Internet domain has a minimum of 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.