The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. In simple terms, the zone is the group of all records for the domain address, so when you open a URL in a web browser, your laptop or 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. With this 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 web site content is requested from the right location, a mail relay server finds out which server handles the e-mails for the domain address (MX record) so a message can be forwarded to the right mailbox, etc. Any change of these sub-records is performed through the company whose name servers are employed, enabling you to keep the website hosting and change only your email provider for instance. Each and every domain address has no less than 2 NS records - primary and secondary, which start with a prefix like NS or DNS.