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 an Internet browser, your laptop or computer asks the DNS servers worldwide where the domain address is hosted and from which servers the DNS records for the domain name ought to be retrieved. With this a web 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 web site content is requested from the correct location, a mail relay server finds out which server manages the emails for the domain (MX record) to ensure that a message can be forwarded to the appropriate mailbox, and so on. Any change of these sub-records is conducted using the company whose name servers are employed, so you're able to keep the website hosting and switch only your email provider for example. Every domain name has no less than two NS records - primary and secondary, which start with a prefix like NS or DNS.