The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain address, so when you open a URL within an Internet browser, your PC asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain name must be retrieved. With this a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP address and the web site content is required from the proper location, a mail relay server discovers which server deals with the e-mails for the domain name (MX record) so a message can be sent to the right mailbox, etc. Any change of these sub-records is conducted with the help of the company whose name servers are used, enabling you to keep the website hosting and change only your email provider for example. Each domain has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.