The NS (Name Server) records of a domain name show 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 within an Internet browser, your PC asks the DNS servers world-wide where the domain address is hosted and from which servers the DNS records for the domain ought to be retrieved. With this a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP and the website content is requested from the proper location, a mail relay server finds out which server manages the e-mails for the domain (MX record) to ensure a message can be forwarded to the right mailbox, etc. Any change of these sub-records is conducted through the company whose name servers are used, so that you can keep the website hosting and change only your email provider for instance. Every single Internet domain has a minimum of 2 NS records - primary and secondary, that start with a prefix like NS or DNS.