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, so when you open a URL inside a browser, your PC asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain name ought to be retrieved. This way a web browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an Internet protocol address and the site content is required from the proper location, a mail relay server discovers which server handles the emails for the domain (MX record) so a message can be sent to the needed mailbox, etc. 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. Every single Internet domain has a minimum of 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.