The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain address, so when you open a URL within a web browser, your laptop or computer asks the DNS servers around the globe where the domain name is hosted and from which servers the DNS records for the domain address should 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 website content is requested from the correct location, a mail relay server finds out which server takes care of the e-mails for the domain address (MX record) so that a message can be sent to the appropriate mailbox, and so forth. Any modification 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 domain address has a minimum of two NS records - primary and secondary, which start with a prefix like NS or DNS.