The NS (Name Server) records of a domain reveal which DNS servers are authoritative for its zone. Simply, the zone is the selection of all records for the domain address, so when you open a URL inside a web browser, your computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain address must be retrieved. 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 correct location, a mail relay server detects which server deals with the e-mails for the domain name (MX record) to ensure a message can be delivered to the right mailbox, and so forth. Any modification of these sub-records is conducted with the help of the company whose name servers are employed, enabling you to keep the website hosting and change only your email provider for instance. Each and every domain has no less than two NS records - primary and secondary, that start with a prefix such as NS or DNS.