The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Simply, the zone is the collection of all records for the domain name, so when you open a URL inside a browser, your laptop or computer asks the DNS servers globally where the domain name is hosted and from which servers the DNS records for the domain name should be retrieved. In this way a web 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 requested from the proper location, a mail relay server finds out which server handles the emails for the domain address (MX record) to ensure a message can be sent to the needed mailbox, etc. Any modification of these sub-records is conducted with the help of the company whose name servers are employed, allowing you to keep the website hosting and switch only your email provider for instance. Every domain name has no less than 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.