The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Simply, the zone is the selection of all records for the domain, so when you open a URL inside an Internet browser, your laptop or computer asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain should be retrieved. That 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 Internet protocol address and the site content is required from the proper location, a mail relay server discovers which server takes care of the e-mails for the domain name (MX record) to ensure that a message can be delivered to the needed mailbox, and so forth. Any modification of these sub-records is performed with the help of the company whose name servers are used, so that you can keep the web hosting and switch only your email provider for example. Each Internet domain has no less than 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.