The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Simply, the zone is the group of all records for the domain name, so when you open a URL in a browser, your laptop or computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain address should be retrieved. With this a web browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP address and the website content is requested from the correct location, a mail relay server detects which server handles the e-mails for the domain address (MX record) to ensure that a message can be delivered to the needed mailbox, and so forth. Any change of these sub-records is done using the company whose name servers are used, so that you can keep the website hosting and switch only your email provider for example. Every domain address has no less than two NS records - primary and secondary, that start with a prefix like NS or DNS.