The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Essentially, the zone is the group of all records for the domain name, so when you open a URL inside a browser, your personal computer asks the DNS servers world-wide where the domain address is hosted and from which servers the DNS records for the domain must be retrieved. 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 website content is requested from the right location, a mail relay server discovers which server handles the e-mails for the domain (MX record) so that a message can be sent to the correct mailbox, and so on. Any change of these sub-records is conducted through the company whose name servers are employed, allowing you to keep the website hosting and switch only your email provider for instance. Each domain address has no less than two NS records - primary and secondary, which start with a prefix such as NS or DNS.