The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Simply, the zone is the group of all records for the domain, so when you open a URL within a web browser, your PC asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain ought to be retrieved. With this a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP and the website content is required from the right location, a mail relay server detects which server takes care of the emails for the domain (MX record) to ensure a message can be sent to the correct mailbox, etc. Any change of these sub-records is performed using the company whose name servers are employed, permitting you to keep the web hosting and switch only your email provider for instance. Each and every domain address has no less than two NS records - primary and secondary, which start with a prefix such as NS or DNS.