The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain, so when you open a URL in a web browser, your personal computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain name ought to be retrieved. In this way a web 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 web site content is requested from the correct location, a mail relay server discovers which server handles the emails for the domain name (MX record) to ensure a message can be delivered to the needed mailbox, etc. Any change of these sub-records is conducted through the company whose name servers are used, so you can keep the website hosting and change only your email provider for instance. Every single domain address has at least 2 NS records - primary and secondary, which start with a prefix like NS or DNS.