The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Basically, the zone is the selection of all records for the domain address, so when you open a URL in a browser, your personal 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 browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP address and the website content is required from the correct location, a mail relay server discovers which server takes care of the e-mails for the domain (MX record) so a message can be delivered to the needed mailbox, etc. Any modification of these sub-records is performed with the help of the company whose name servers are used, so that you can keep the website hosting and change only your email provider for instance. Each Internet domain has no less than 2 NS records - primary and secondary, that start with a prefix like NS or DNS.