The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain address, so when you open a URL in a web browser, your laptop or computer asks the DNS servers globally where the domain name is hosted and from which servers the DNS records for the domain should be retrieved. In this way a 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 required from the correct location, a mail relay server finds out which server deals with the e-mails for the domain address (MX record) to ensure that a message can be sent to the correct mailbox, etc. Any change of these sub-records is conducted using the company whose name servers are employed, permitting you to keep the website hosting and change only your email provider for example. Every domain address has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.