The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Simply, the zone is the collection of all records for the domain address, so when you open a URL within a browser, your laptop or computer asks the DNS servers world-wide where the domain address is hosted and from which servers the DNS records for the domain name must be retrieved. With this a web 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 website content is requested from the proper location, a mail relay server finds out which server handles the e-mails for the domain address (MX record) to ensure that a message can be delivered to the right mailbox, and so on. Any modification of these sub-records is conducted with the help of the company whose name servers are employed, so that you can keep the web hosting and change only your email provider for instance. Each domain has no less than 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.