The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Essentially, the zone is the selection of all records for the domain, so when you open a URL inside a web browser, your laptop or computer asks the DNS servers globally where the domain address is hosted and from which servers the DNS records for the domain ought to be retrieved. This way a web 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 proper location, a mail relay server detects which server handles the emails for the domain (MX record) to ensure that a message can be delivered to the appropriate mailbox, and so on. Any change of these sub-records is conducted through the company whose name servers are used, so that you can keep the web hosting and switch only your email provider for instance. Each and every domain address has no less than two NS records - primary and secondary, that start with a prefix like NS or DNS.