The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain, so when you open a URL in a web browser, your personal computer asks the DNS servers around the globe where the domain name 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 is so that the latter is mapped to an IP and the website content is required from the correct location, a mail relay server detects which server manages the emails for the domain name (MX record) to ensure that a message can be forwarded to the appropriate mailbox, and so on. Any change of these sub-records is performed through the company whose name servers are used, permitting you to keep the web hosting and change only your email provider for instance. Each and every Internet domain has a minimum of 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.