The NS (Name Server) records of a domain name point out 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 in a web browser, your computer asks the DNS servers globally where the domain 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 address 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 manages the emails for the domain address (MX record) so a message can be sent to the needed mailbox, etc. Any change of these sub-records is done through the company whose name servers are used, so that you can keep the website hosting and switch only your email provider for instance. Every domain address has a minimum of two NS records - primary and secondary, which start with a prefix such as NS or DNS.