The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Basically, the zone is the group of all records for the domain name, so when you open a URL in a web browser, your personal computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain address ought to be retrieved. This way a web browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the site content is requested from the correct location, a mail relay server discovers which server deals with the emails for the domain name (MX record) to ensure that a message can be sent to the right mailbox, and so on. Any change of these sub-records is conducted with the help of the company whose name servers are employed, allowing you to keep the web hosting and change only your email provider for instance. Each and every Internet domain has a minimum of two NS records - primary and secondary, which start with a prefix such as NS or DNS.