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 within a web browser, your PC asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain should be retrieved. In this way a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an Internet protocol address and the website content is required from the correct location, a mail relay server detects which server deals with the e-mails for the domain (MX record) so that a message can be sent to the appropriate mailbox, and so forth. Any modification of these sub-records is performed through the company whose name servers are employed, allowing you to keep the web hosting and switch only your email provider for example. Every single Internet domain has a minimum of 2 NS records - primary and secondary, which start with a prefix like NS or DNS.