The NS (Name Server) records of a domain show 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 browser, your laptop or computer asks the DNS servers worldwide where the domain name is hosted and from which servers the DNS records for the domain must 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 Internet protocol address and the website content is requested from the correct location, a mail relay server finds out which server handles the emails for the domain name (MX record) so that a message can be forwarded to the appropriate mailbox, and so on. Any modification of these sub-records is done using the company whose name servers are employed, so that you can keep the web hosting and switch only your email provider for instance. Every single domain has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.