The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Simply, the zone is the group of all records for the domain, so when you open a URL inside an Internet browser, your PC asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain should be retrieved. That 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 Internet protocol address and the website content is required from the right location, a mail relay server discovers which server manages the emails for the domain name (MX record) so a message can be delivered to the right mailbox, and so forth. Any change of these sub-records is done through the company whose name servers are employed, permitting you to keep the web hosting and switch only your email provider for instance. Every domain name has at least two NS records - primary and secondary, which start with a prefix like NS or DNS.