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 address, so when you open a URL in a web browser, your PC asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain ought to 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 correct location, a mail relay server detects which server handles the e-mails for the domain address (MX record) to ensure a message can be sent to the right mailbox, and so forth. Any modification of these sub-records is performed through the company whose name servers are employed, so you're able to keep the web hosting and change only your email provider for example. Every Internet domain has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.