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 browser, your PC asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain address must be retrieved. That way a browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP and the website content is requested from the correct location, a mail relay server detects which server takes care of the e-mails for the domain (MX record) so that a message can be delivered to the right mailbox, and so on. Any modification of these sub-records is conducted using the company whose name servers are used, permitting you to keep the web hosting and change only your email provider for example. Every Internet domain has at least 2 NS records - primary and secondary, that start with a prefix like NS or DNS.