The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Simply, the zone is the group of all records for the domain, so when you open a URL within a web browser, your computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain name should be retrieved. In this way a browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP and the website content is required from the proper location, a mail relay server finds out which server handles the emails for the domain name (MX record) to ensure that a message can be sent to the right mailbox, and so forth. Any change of these sub-records is performed with the help of 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 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.