The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. In simple terms, the zone is the group of all records for the domain, so when you open a URL in a 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 website content is requested from the proper location, a mail relay server detects which server manages the emails for the domain address (MX record) to ensure that a message can be forwarded to the right mailbox, and so on. Any change of these sub-records is done through the company whose name servers are used, so you're able to keep the website hosting and change only your email provider for instance. Every single domain name has a minimum of 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.