The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Essentially, the zone is the range of all records for the domain address, so when you open a URL inside a browser, your personal computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain should be retrieved. That way a browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP address and the website content is required from the correct location, a mail relay server discovers which server handles the e-mails for the domain name (MX record) so a message can be sent to the right mailbox, etc. Any modification of these sub-records is conducted through the company whose name servers are used, so that you can keep the web hosting and change only your email provider for example. Every Internet domain has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.