The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Essentially, the zone is the group of all records for the domain address, so when you open a URL within a web browser, your computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain name should be retrieved. This way a 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 web site content is required from the correct location, a mail relay server detects which server deals with the emails for the domain name (MX record) to ensure a message can be sent to the appropriate mailbox, etc. Any change of these sub-records is performed using the company whose name servers are used, permitting you to keep the web hosting and switch only your email provider for instance. Each and every domain name has a minimum of two NS records - primary and secondary, which start with a prefix such as NS or DNS.