The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. In simple terms, the zone is the selection of all records for the domain name, so when you open a URL inside a browser, your PC asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain name must be retrieved. In 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 Internet protocol address and the website content is requested from the correct location, a mail relay server finds out which server deals with the emails for the domain name (MX record) so that a message can be delivered to the right mailbox, and so forth. Any modification of these sub-records is done using the company whose name servers are used, permitting you to keep the web hosting and change only your email provider for example. Every Internet domain has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.