The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Basically, the zone is the range of all records for the domain, so when you open a URL in a browser, your laptop or computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain name must be retrieved. That 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 and the site content is requested from the correct location, a mail relay server detects which server handles the e-mails for the domain address (MX record) so that a message can be forwarded to the appropriate mailbox, and so forth. Any modification of these sub-records is done with the help of the company whose name servers are used, allowing you to keep the web hosting and change 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.