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 a web browser, your personal computer asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain name must be retrieved. This way a browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP and the web site content is required from the proper location, a mail relay server discovers which server takes care of the emails for the domain name (MX record) so a message can be sent to the right mailbox, and so on. Any modification of these sub-records is conducted through the company whose name servers are employed, permitting you to keep the website hosting and switch only your email provider for instance. Every single domain name has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.