The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Basically, the zone is the collection 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 name is hosted and from which servers the DNS records for the domain must 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 Internet protocol address and the site content is requested from the correct location, a mail relay server detects which server deals with the e-mails for the domain address (MX record) to ensure that a message can be delivered to the correct mailbox, etc. Any modification of these sub-records is performed with the help of the company whose name servers are employed, permitting you to keep the web hosting and switch only your email provider for example. Every domain name has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.