The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Essentially, the zone is the group of all records for the domain, so when you open a URL in a browser, your PC asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain must be retrieved. That way a web browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the site content is required from the proper location, a mail relay server finds out which server deals with the emails for the domain (MX record) to ensure a message can be delivered to the appropriate mailbox, and so on. Any change of these sub-records is done with the help of the company whose name servers are employed, allowing you to keep the web hosting and switch only your email provider for instance. Every Internet domain has no less than two NS records - primary and secondary, that start with a prefix like NS or DNS.