The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Basically, the zone is the collection of all records for the domain name, so when you open a URL in 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 must be retrieved. In this 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 required from the right location, a mail relay server finds out which server handles the emails for the domain (MX record) to ensure that a message can be forwarded to the right mailbox, and so forth. Any change of these sub-records is conducted using the company whose name servers are employed, permitting you to keep the website hosting and change only your email provider for instance. Every single Internet domain has at least 2 NS records - primary and secondary, that start with a prefix like NS or DNS.