The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain name, so when you open a URL inside an Internet browser, your PC asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain must be retrieved. That way a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an Internet protocol address and the website content is requested from the proper location, a mail relay server detects which server handles the e-mails for the domain name (MX record) so that a message can be delivered to the right mailbox, and so on. Any change of these sub-records is conducted with the help of the company whose name servers are employed, permitting you to keep the website hosting and switch only your email provider for instance. Each and every Internet domain has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.