The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Essentially, the zone is the selection of all records for the domain name, so when you open a URL inside an Internet browser, your PC asks the DNS servers worldwide where the domain is hosted and from which servers the DNS records for the domain name ought to be retrieved. In this way a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an IP address and the web site content is required from the proper location, a mail relay server detects which server takes care of the e-mails for the domain address (MX record) so that a message can be sent to the correct mailbox, and so on. Any modification of these sub-records is performed through the company whose name servers are used, so you can keep the website hosting and switch only your email provider for example. Each domain has a minimum of 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.