The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Essentially, the zone is the range of all records for the domain address, so when you open a URL in a browser, your laptop or computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain ought to be retrieved. That way a web 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 website content is requested from the proper location, a mail relay server discovers which server manages the e-mails for the domain address (MX record) to ensure that a message can be sent to the right mailbox, and so on. Any change of these sub-records is conducted with the help of the company whose name servers are employed, permitting you to keep the web hosting and change only your email provider for example. Every single domain address has a minimum of two NS records - primary and secondary, which start with a prefix like NS or DNS.