The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Simply, the zone is the range of all records for the domain name, so when you open a URL in an Internet browser, your computer asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain name must be retrieved. This 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 proper location, a mail relay server discovers which server manages the emails for the domain name (MX record) to ensure a message can be delivered to the appropriate mailbox, and so on. Any change of these sub-records is performed through the company whose name servers are used, so you're able to keep the website hosting and switch only your email provider for example. Each domain address has no less than two NS records - primary and secondary, that start with a prefix such as NS or DNS.