The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain address, so when you open a URL inside a web 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. That way a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP and the site content is requested from the right location, a mail relay server discovers which server deals with the emails for the domain (MX record) so that a message can be delivered to the correct mailbox, and so on. Any modification of these sub-records is performed through the company whose name servers are used, enabling you to keep the web hosting and switch only your email provider for instance. Every domain address has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.