The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. In simple terms, the zone is the range of all records for the domain address, so when you open a URL in an Internet browser, your laptop or computer asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain name should be retrieved. This 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 IP address and the website content is requested from the correct location, a mail relay server finds out which server handles the emails for the domain address (MX record) to ensure that a message can be sent to the right mailbox, and so on. Any modification of these sub-records is performed with the help of the company whose name servers are used, so you can keep the website hosting and change only your email provider for instance. Each and every domain address has no less than 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.