The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Simply, the zone is the range of all records for the domain name, so when you open a URL within a browser, your computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain address ought to be retrieved. 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 website content is required from the right location, a mail relay server discovers which server takes care of the e-mails for the domain (MX record) so a message can be forwarded to the appropriate mailbox, and so forth. Any modification of these sub-records is done with the help of the company whose name servers are used, enabling you to keep the web hosting and switch only your email provider for instance. Each and every Internet domain has a minimum of 2 NS records - primary and secondary, that start with a prefix like NS or DNS.