The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain, so when you open a URL inside a web browser, your computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain name ought to be retrieved. That way a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the web site content is required from the right location, a mail relay server finds out which server handles the emails for the domain (MX record) to ensure that a message can be delivered to the correct mailbox, and so forth. Any change 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 change only your email provider for instance. Every domain has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.