The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Basically, the zone is the group of all records for the domain address, so when you open a URL within a web browser, your laptop or computer asks the DNS servers around the globe where the domain name is hosted and from which servers the DNS records for the domain address must be retrieved. 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 Internet protocol address and the site content is requested from the right location, a mail relay server finds out which server takes care of the emails for the domain name (MX record) so that a message can be sent to the right mailbox, and so on. Any change of these sub-records is done through the company whose name servers are employed, permitting you to keep the website hosting and switch only your email provider for instance. Each domain address has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.