The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the selection of all records for the domain address, so when you open a URL within a web browser, your PC asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain address should be retrieved. That 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 manages the e-mails for the domain (MX record) to ensure that a message can be forwarded to the right mailbox, and so on. Any change of these sub-records is performed using the company whose name servers are used, allowing you to keep the website hosting and change only your email provider for example. Every domain has at least two NS records - primary and secondary, that start with a prefix like NS or DNS.