The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Essentially, the zone is the selection of all records for the domain address, so when you open a URL inside a browser, your computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain should be retrieved. With this a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP and the website content is required from the correct location, a mail relay server detects which server manages the e-mails for the domain name (MX record) so that a message can be sent to the appropriate mailbox, and so forth. Any change of these sub-records is done using the company whose name servers are used, so you're able to keep the web hosting and change only your email provider for instance. Each domain name has at least 2 NS records - primary and secondary, that start with a prefix such as NS or DNS.