The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain address, so when you open a URL inside a web browser, your personal computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain name ought to be retrieved. This way a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP and the site content is requested from the correct location, a mail relay server detects which server takes care of the emails for the domain name (MX record) to ensure a message can be forwarded to the correct mailbox, and so on. Any modification of these sub-records is conducted using the company whose name servers are used, allowing you to keep the website hosting and switch only your email provider for instance. Each Internet domain has no less than 2 NS records - primary and secondary, which start with a prefix like NS or DNS.