The NS (Name Server) records of a domain name show which DNS servers are authoritative for its zone. Basically, the zone is the group of all records for the domain address, so when you open a URL in a web browser, your laptop or computer asks the DNS servers world-wide where the domain name is hosted and from which servers the DNS records for the domain name ought to be retrieved. This 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 and the site content is required from the right location, a mail relay server detects which server handles the emails for the domain address (MX record) to ensure a message can be forwarded to the appropriate mailbox, and so forth. Any change of these sub-records is conducted with the help of the company whose name servers are employed, so that you can keep the web hosting and switch only your email provider for example. Every single domain name has at least 2 NS records - primary and secondary, that start with a prefix like NS or DNS.