The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Simply, the zone is the group of all records for the domain address, so when you open a URL in a web browser, your PC asks the DNS servers worldwide where the domain address is hosted and from which servers the DNS records for the domain address must be retrieved. With this 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 proper location, a mail relay server discovers which server takes care of the emails for the domain (MX record) to ensure that a message can be forwarded 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 used, so you're able to keep the web hosting and change only your email provider for example. Every single domain name has no less than 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.