The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. In simple terms, the zone is the selection 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 worldwide where the domain name is hosted and from which servers the DNS records for the domain address should be retrieved. That 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 site content is requested from the correct location, a mail relay server detects which server takes care of the e-mails for the domain name (MX record) to ensure a message can be forwarded to the needed mailbox, and so forth. Any modification of these sub-records is done using the company whose name servers are used, enabling you to keep the web hosting and switch only your email provider for instance. Each and every domain name has a minimum of 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.