The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Essentially, the zone is the group of all records for the domain, so when you open a URL within an Internet browser, your PC asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain name ought to 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 required from the right location, a mail relay server finds out which server deals with the emails for the domain address (MX record) so that a message can be forwarded to the correct mailbox, and so forth. Any change of these sub-records is performed using 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 no less than two NS records - primary and secondary, that start with a prefix like NS or DNS.