The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. In simple terms, the zone is the group of all records for the domain name, so when you open a URL within a browser, your personal computer 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. That way a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the web site content is requested from the correct location, a mail relay server detects which server manages the e-mails for the domain name (MX record) so that a message can be sent to the correct mailbox, and so on. Any modification of these sub-records is done with the help of the company whose name servers are used, so that you can keep the web hosting and change only your email provider for example. Each domain address has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.