The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Simply, the zone is the range of all records for the domain address, so when you open a URL within a browser, your personal computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain address ought to be retrieved. 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 discovers which server handles the e-mails for the domain address (MX record) to ensure that a message can be delivered to the appropriate mailbox, etc. Any change of these sub-records is performed with the help of the company whose name servers are used, allowing you to keep the web hosting and change only your email provider for instance. Every single domain address has no less than two NS records - primary and secondary, that start with a prefix such as NS or DNS.