The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Essentially, the zone is the range of all records for the domain, so when you open a URL inside a web browser, your personal computer asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain name must be retrieved. This 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 address and the website content is requested from the proper location, a mail relay server finds out which server deals with the emails for the domain address (MX record) to ensure that a message can be sent to the needed mailbox, and so forth. Any change of these sub-records is performed using the company whose name servers are employed, allowing you to keep the web hosting and change only your email provider for example. Every single Internet domain has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.