The NS (Name Server) records of a domain show 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 inside an Internet browser, your computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain ought to be retrieved. With this a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the website content is requested from the right location, a mail relay server discovers which server handles the emails for the domain (MX record) to ensure that a message can be forwarded to the needed mailbox, etc. Any change of these sub-records is performed through the company whose name servers are used, so that you can keep the web hosting and change only your email provider for instance. Each domain name has a minimum of 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.