The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Basically, the zone is the group of all records for the domain, so when you open a URL within a web browser, your personal 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. In 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 web site content is requested from the correct location, a mail relay server finds out which server deals with the e-mails for the domain name (MX record) so a message can be forwarded to the appropriate mailbox, etc. Any modification of these sub-records is done through the company whose name servers are used, permitting you to keep the web hosting and switch only your email provider for instance. Each and every domain name has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.