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 inside 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 ought to be retrieved. That way a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP and the site content is requested from the proper location, a mail relay server discovers which server manages the e-mails for the domain name (MX record) to ensure that a message can be sent to the correct mailbox, etc. Any modification of these sub-records is performed with the help of the company whose name servers are used, allowing you to keep the website hosting and switch only your email provider for instance. Each Internet domain has at least 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.