The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain name, so when you open a URL in a web browser, your personal computer asks the DNS servers world-wide where the domain address is hosted and from which servers the DNS records for the domain ought to be retrieved. That 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 address and the web site content is required from the right location, a mail relay server finds out which server deals with the e-mails for the domain (MX record) so a message can be sent to the correct mailbox, and so on. Any modification of these sub-records is performed with the help of the company whose name servers are employed, so that you can keep the web hosting and change only your email provider for example. Every domain has a minimum of 2 NS records - primary and secondary, which start with a prefix such as NS or DNS.