The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the range of all records for the domain address, so when you open a URL inside a browser, your PC asks the DNS servers globally where the domain is hosted and from which servers the DNS records for the domain 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 web site content is required from the correct location, a mail relay server finds out which server manages the emails for the domain name (MX record) so a message can be delivered to the right mailbox, and so forth. Any change of these sub-records is done with the help of the company whose name servers are used, so you can keep the website hosting and change only your email provider for instance. Each and every domain name has no less than 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.