The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain address, so when you open a URL inside a browser, your personal computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain address should be retrieved. With this a browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP and the website content is requested from the right location, a mail relay server discovers which server deals with the emails for the domain address (MX record) to ensure that a message can be forwarded to the appropriate mailbox, and so forth. Any modification of these sub-records is performed using the company whose name servers are used, so that you can keep the website hosting and change only your email provider for example. Each domain name has at least 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.