The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain address, so when you open a URL within an Internet browser, your PC asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain should be retrieved. In 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 Internet protocol address and the website content is requested from the right location, a mail relay server discovers which server deals with the emails for the domain name (MX record) so that a message can be sent to the appropriate mailbox, and so on. Any modification of these sub-records is done using the company whose name servers are employed, so that you can keep the website hosting and switch only your email provider for example. Each domain has at least two NS records - primary and secondary, which start with a prefix like NS or DNS.