The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain, so when you open a URL in an Internet browser, your PC asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain name must 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 IP address and the website content is required from the proper location, a mail relay server finds out which server manages the emails for the domain address (MX record) to ensure that a message can be sent to the appropriate mailbox, etc. Any change of these sub-records is done using the company whose name servers are employed, permitting you to keep the web hosting and change only your email provider for instance. Each and every Internet domain has at least two NS records - primary and secondary, which start with a prefix like NS or DNS.