The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Simply, the zone is the group of all records for the domain address, so when you open a URL in an Internet browser, your personal computer asks the DNS servers globally where the domain address 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 name is so that the latter is mapped to an IP address and the web site content is required from the proper location, a mail relay server finds out which server handles the e-mails for the domain (MX record) to ensure a message can be sent to the right mailbox, and so forth. Any change of these sub-records is conducted with the help of the company whose name servers are employed, permitting you to keep the web hosting and switch only your email provider for instance. Every single domain has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.