The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. In simple terms, the zone is the selection of all records for the domain name, so when you open a URL within a web browser, your laptop or computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain address must be retrieved. With this a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an Internet protocol address and the web site content is required from the correct location, a mail relay server finds out which server manages the e-mails for the domain address (MX record) so a message can be delivered to the appropriate mailbox, and so forth. Any modification of these sub-records is done using the company whose name servers are used, so that you can keep the web hosting and switch only your email provider for instance. Every single domain address has at least two NS records - primary and secondary, which start with a prefix such as NS or DNS.