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, so when you open a URL inside an Internet browser, your personal computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain ought to be retrieved. This 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 address and the website content is requested from the right location, a mail relay server finds out which server manages the emails for the domain (MX record) so that a message can be delivered to the correct mailbox, etc. Any modification of these sub-records is done with the help of the company whose name servers are employed, so you can keep the website hosting and change only your email provider for instance. Each and every domain address has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.