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 range of all records for the domain, so when you open a URL within an Internet browser, your personal computer asks the DNS servers world-wide where the domain address is hosted and from which servers the DNS records for the domain address ought to be retrieved. In this way a 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 site content is required from the right location, a mail relay server finds out which server takes care of the emails for the domain address (MX record) so that a message can be sent to the right mailbox, and so forth. Any modification of these sub-records is performed using the company whose name servers are used, permitting you to keep the website hosting and change only your email provider for instance. Every domain address has at least two NS records - primary and secondary, that start with a prefix like NS or DNS.