The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Simply, the zone is the range of all records for the domain address, so when you open a URL within an Internet browser, your laptop or computer asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain address should be retrieved. This 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 finds out which server takes care of the e-mails for the domain address (MX record) so that a message can be forwarded to the appropriate mailbox, etc. Any modification of these sub-records is conducted with the help of the company whose name servers are employed, enabling you to keep the web hosting and change only your email provider for instance. Each domain address has at least 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.