The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Basically, the zone is the collection of all records for the domain address, so when you open a URL in a browser, your personal computer asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain address must be retrieved. With this 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 website content is requested from the right location, a mail relay server finds out which server deals with the emails for the domain address (MX record) so that a message can be delivered to the right mailbox, and so forth. Any modification of these sub-records is conducted with the help of the company whose name servers are used, permitting you to keep the website hosting and change only your email provider for instance. Every single domain has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.