The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain address, so when you open a URL in a web browser, your personal computer asks the DNS servers globally where the domain name is hosted and from which servers the DNS records for the domain must be retrieved. That way a web browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the web site content is requested from the right location, a mail relay server detects which server takes care of the e-mails for the domain name (MX record) so a message can be sent to the right mailbox, and so on. Any change of these sub-records is done through the company whose name servers are employed, enabling you to keep the website hosting and switch only your email provider for instance. Every single domain name has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.
