The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Simply, 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 globally where the domain address is hosted and from which servers the DNS records for the domain address should be retrieved. That way a browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP address and the site content is requested from the right location, a mail relay server discovers which server takes care of the e-mails for the domain name (MX record) to ensure a message can be sent to the appropriate mailbox, and so forth. Any modification of these sub-records is conducted using the company whose name servers are used, so you're able to keep the web hosting and change only your email provider for example. Every Internet domain has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.