The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Simply, the zone is the range of all records for the domain, so when you open a URL inside a web browser, your PC asks the DNS servers globally where the domain name is hosted and from which servers the DNS records for the domain name should be retrieved. This way a web 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 web site content is required from the proper location, a mail relay server discovers which server handles the emails for the domain (MX record) to ensure a message can be sent 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 web hosting and change only your email provider for example. Each domain address has a minimum of 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.