The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Simply, the zone is the group of all records for the domain, so when you open a URL in a web browser, your PC asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain name must be retrieved. This way 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 site content is required from the correct location, a mail relay server discovers which server handles the e-mails for the domain (MX record) to ensure a message can be forwarded to the needed mailbox, and so on. Any change of these sub-records is conducted through the company whose name servers are employed, permitting you to keep the website hosting and switch only your email provider for example. Every single domain name has a minimum of 2 NS records - primary and secondary, that start with a prefix like NS or DNS.