The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Essentially, the zone is the selection of all records for the domain name, so when you open a URL within a browser, your laptop or computer asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain address must be retrieved. In 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 Internet protocol address and the website content is requested from the right location, a mail relay server detects which server deals with the e-mails for the domain address (MX record) to ensure that a message can be delivered to the needed mailbox, etc. 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 domain name has a minimum of 2 NS records - primary and secondary, which start with a prefix like NS or DNS.