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 name, so when you open a URL within a browser, your computer asks the DNS servers world-wide 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 address is so that the latter is mapped to an IP and the website content is requested from the correct location, a mail relay server discovers which server manages the e-mails for the domain (MX record) to ensure that a message can be delivered to the right mailbox, and so forth. Any modification of these sub-records is performed with the help of the company whose name servers are employed, so you're able to keep the website hosting and switch only your email provider for instance. Every domain name has a minimum of 2 NS records - primary and secondary, that start with a prefix like NS or DNS.