The NS (Name Server) records of a domain reveal 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 in an Internet browser, your personal computer asks the DNS servers world-wide where the domain address is hosted and from which servers the DNS records for the domain address must be retrieved. With this 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 required from the proper location, a mail relay server finds out which server deals with the emails for the domain name (MX record) to ensure a message can be delivered to the right mailbox, etc. Any change of these sub-records is done with the help of the company whose name servers are employed, so you're able to keep the web hosting and switch only your email provider for instance. Each and every domain address has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.