The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain name, so when you open a URL inside a browser, your computer asks the DNS servers world-wide where the domain address is hosted and from which servers the DNS records for the domain name must be retrieved. With this 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 and the website content is requested from the proper location, a mail relay server detects which server manages the emails for the domain address (MX record) so that a message can be sent to the needed mailbox, and so on. Any modification of these sub-records is done with the help of the company whose name servers are used, allowing you to keep the web hosting and switch only your email provider for instance. Each domain has no less than 2 NS records - primary and secondary, that start with a prefix like NS or DNS.