The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. In simple terms, the zone is the range of all records for the domain, so when you open a URL in an Internet browser, your computer asks the DNS servers globally where the domain name is hosted and from which servers the DNS records for the domain address ought to be retrieved. In this way a 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 requested from the right location, a mail relay server discovers which server manages the e-mails for the domain (MX record) to ensure that a message can be sent to the right mailbox, and so on. Any modification of these sub-records is done with the help of the company whose name servers are employed, allowing you to keep the web hosting and switch only your email provider for instance. Every single domain name has no less than two NS records - primary and secondary, that start with a prefix such as NS or DNS.