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 in a web browser, your PC 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. In this way a web browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP address and the site content is required from the right location, a mail relay server finds out which server handles the emails for the domain (MX record) so a message can be delivered to the correct mailbox, etc. Any modification of these sub-records is done through the company whose name servers are used, so that you can keep the web hosting and switch only your email provider for instance. Every domain has at least two NS records - primary and secondary, that start with a prefix like NS or DNS.