The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Simply, the zone is the selection of all records for the domain name, so when you open a URL in a browser, your laptop or computer asks the DNS servers around the globe where the domain name is hosted and from which servers the DNS records for the domain name should 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 Internet protocol address and the web site content is requested from the proper location, a mail relay server finds out which server deals with the e-mails for the domain (MX record) so a message can be sent to the appropriate mailbox, and so on. Any modification of these sub-records is conducted through the company whose name servers are used, so that you can keep the web hosting and change only your email provider for example. Every domain name has no less than 2 NS records - primary and secondary, which start with a prefix like NS or DNS.