The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Essentially, the zone is the selection of all records for the domain, so when you open a URL inside an Internet browser, your personal computer asks the DNS servers worldwide 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 name 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 handles the e-mails for the domain (MX record) so that a message can be sent to the appropriate mailbox, etc. Any modification of these sub-records is done using the company whose name servers are used, so you can keep the web hosting and switch only your email provider for example. Every single domain address has no less than two NS records - primary and secondary, which start with a prefix such as NS or DNS.