The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain address, so when you open a URL inside a browser, your computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain should 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 and the web site content is required from the correct location, a mail relay server discovers which server handles the emails for the domain address (MX record) so a message can be delivered to the appropriate mailbox, and so forth. Any change of these sub-records is performed with the help of the company whose name servers are employed, enabling you to keep the web hosting and switch only your email provider for example. Each Internet domain has at least 2 NS records - primary and secondary, which start with a prefix like NS or DNS.