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 address, so when you open a URL inside an Internet browser, your PC asks the DNS servers globally where the domain name is hosted and from which servers the DNS records for the domain name should 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 right location, a mail relay server discovers which server manages the emails for the domain address (MX record) so that a message can be forwarded to the needed mailbox, etc. Any change of these sub-records is performed using the company whose name servers are employed, so that you can keep the web hosting and switch only your email provider for instance. Each and every domain name has at least two NS records - primary and secondary, that start with a prefix like NS or DNS.