The NS (Name Server) records of a domain 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 inside a web 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 web 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 site content is requested from the correct location, a mail relay server finds out which server handles the e-mails for the domain (MX record) so a message can be forwarded to the right mailbox, and so forth. Any change of these sub-records is performed through the company whose name servers are employed, so you're able to keep the website hosting and change only your email provider for example. Each and every domain address has a minimum of 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.