The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Essentially, the zone is the group of all records for the domain, so when you open a URL within a web browser, your computer asks the DNS servers around the globe where the domain 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 address is so that the latter is mapped to an IP address and the web site content is requested from the proper location, a mail relay server discovers which server manages the emails for the domain name (MX record) to ensure a message can be sent to the needed mailbox, and so on. Any modification of these sub-records is done through the company whose name servers are employed, allowing you to keep the web hosting and switch only your email provider for example. Each and every domain address has no less than 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.
