The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Basically, the zone is the collection of all records for the domain address, 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 must 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 web site content is requested from the proper location, a mail relay server discovers which server manages the emails for the domain name (MX record) to ensure a message can be delivered to the needed mailbox, etc. Any change of these sub-records is conducted using the company whose name servers are employed, permitting you to keep the web hosting and switch only your email provider for example. Each domain name has a minimum of two NS records - primary and secondary, which start with a prefix such as NS or DNS.