The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Essentially, the zone is the group of all records for the domain name, so when you open a URL inside a browser, your personal computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain name must 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 right location, a mail relay server detects which server handles the emails for the domain (MX record) to ensure that a message can be forwarded to the needed mailbox, and so forth. Any change of these sub-records is done using the company whose name servers are used, enabling you to keep the web hosting and change only your email provider for instance. Each and every domain name has at least 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.