The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Essentially, the zone is the selection of all records for the domain, so when you open a URL in a browser, your laptop or computer asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain name must be retrieved. In this 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 website content is requested from the right location, a mail relay server discovers which server takes care of the emails for the domain address (MX record) so a message can be forwarded to the needed mailbox, and so forth. Any change of these sub-records is conducted 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. Every single domain has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.