The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Essentially, the zone is the group of all records for the domain, so when you open a URL in a web browser, your computer asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain name ought to be retrieved. In this way a web 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 web site content is requested from the right location, a mail relay server finds out which server handles the emails for the domain address (MX record) to ensure a message can be forwarded to the correct mailbox, etc. Any change of these sub-records is performed using the company whose name servers are employed, so that you can keep the web hosting and switch only your email provider for example. Every domain has at least two NS records - primary and secondary, which start with a prefix like NS or DNS.