The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain name, so when you open a URL inside a web 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 address must be retrieved. In this way a browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP address and the site content is requested from the right location, a mail relay server discovers which server takes care of the e-mails for the domain address (MX record) so that a message can be delivered to the appropriate mailbox, and so on. Any change of these sub-records is done with the help of the company whose name servers are used, so you can keep the web hosting and change only your email provider for example. Every single domain address has at least 2 NS records - primary and secondary, which start with a prefix like NS or DNS.