The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain, so when you open a URL within a web browser, your laptop or computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain address ought to be retrieved. That 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 discovers which server deals with the emails for the domain name (MX record) to ensure that a message can be sent to the appropriate mailbox, and so forth. Any modification of these sub-records is conducted through the company whose name servers are used, allowing you to keep the web hosting and switch only your email provider for example. Each and every domain name has at least 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.