The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Essentially, the zone is the group 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 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 name is so that the latter is mapped to an IP address and the website content is requested from the right location, a mail relay server discovers which server manages the e-mails for the domain address (MX record) to ensure a message can be delivered to the right 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 website hosting and change only your email provider for instance. Each Internet domain has no less than two NS records - primary and secondary, that start with a prefix such as NS or DNS.