The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. In simple terms, the zone is the group of all records for the domain address, so when you open a URL inside a browser, your personal computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain address should 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 site content is requested from the right location, a mail relay server finds out which server deals with the e-mails for the domain address (MX record) to ensure a message can be forwarded to the needed mailbox, etc. Any change of these sub-records is conducted through the company whose name servers are used, permitting you to keep the website hosting and change only your email provider for example. Every single domain has a minimum of two NS records - primary and secondary, that start with a prefix such as NS or DNS.