The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain, so when you open a URL in a browser, your PC asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain should be retrieved. In this way a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an Internet protocol address and the website content is requested from the proper location, a mail relay server finds out which server deals with the emails for the domain name (MX record) so a message can be forwarded to the right mailbox, etc. Any modification of these sub-records is conducted through the company whose name servers are employed, enabling you to keep the web hosting and switch only your email provider for example. Every domain has a minimum of 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.