The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Simply, the zone is the collection of all records for the domain address, so when you open a URL inside a web browser, your PC asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain name ought to be retrieved. That way a 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 web site content is requested from the correct location, a mail relay server detects which server takes care of the e-mails for the domain address (MX record) to ensure that a message can be delivered to the needed mailbox, and so on. Any modification of these sub-records is done with the help of the company whose name servers are employed, so you can keep the web hosting and change only your email provider for example. Each and every domain name has no less than 2 NS records - primary and secondary, that start with a prefix like NS or DNS.