The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Simply, the zone is the range of all records for the domain name, so when you open a URL in a web browser, your personal computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain address ought to be retrieved. In 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 Internet protocol address and the website content is required from the right location, a mail relay server finds out which server deals with the emails for the domain address (MX record) to ensure a message can be sent to the right mailbox, and so forth. Any change of these sub-records is done with the help of the company whose name servers are employed, so that you can keep the website hosting and switch only your email provider for example. Each Internet domain has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.