The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Essentially, the zone is the range of all records for the domain, so when you open a URL inside an Internet browser, your laptop or computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain address must 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 requested from the proper location, a mail relay server finds out which server takes care of the e-mails for the domain (MX record) so a message can be delivered to the right mailbox, and so on. Any modification of these sub-records is performed with the help of the company whose name servers are employed, permitting you to keep the web hosting and switch only your email provider for instance. Every domain name has at least two NS records - primary and secondary, that start with a prefix such as NS or DNS.