The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain name, so when you open a URL in a browser, your computer asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain address should be retrieved. With this a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an Internet protocol address and the site content is required from the proper location, a mail relay server detects which server deals with the emails for the domain (MX record) to ensure a message can be sent to the right mailbox, and so forth. Any change of these sub-records is done through the company whose name servers are employed, allowing you to keep the web hosting and change only your email provider for instance. Each and every domain address has at least 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.