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 name, so when you open a URL in an Internet browser, your personal computer asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain name must be retrieved. That way a 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 site content is requested from the proper location, a mail relay server detects which server deals with the e-mails for the domain name (MX record) so that a message can be sent to the right mailbox, and so forth. Any modification of these sub-records is done using the company whose name servers are used, permitting you to keep the website hosting and switch only your email provider for example. Each Internet domain has at least two NS records - primary and secondary, that start with a prefix like NS or DNS.