The NS (Name Server) records of a domain name show 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 within a browser, your laptop or computer asks the DNS servers around the globe where the domain address is hosted and from which servers the DNS records for the domain address must be retrieved. In this way a web browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP and the web site content is requested from the proper location, a mail relay server finds out which server deals with the emails for the domain address (MX record) so a message can be forwarded to the appropriate mailbox, and so forth. Any modification of these sub-records is conducted with the help of the company whose name servers are used, permitting you to keep the web hosting and change only your email provider for instance. Each domain has no less than 2 NS records - primary and secondary, which start with a prefix like NS or DNS.