The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain address, so when you open a URL within a web browser, your PC asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain address ought to be retrieved. With this a browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP address and the web site content is required from the correct location, a mail relay server finds out which server deals with the emails for the domain (MX record) so that a message can be sent to the appropriate mailbox, etc. Any modification of these sub-records is performed with the help of the company whose name servers are employed, enabling you to keep the website hosting and change only your email provider for example. Each domain address has at least 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.