The NS (Name Server) records of a domain name point out which DNS servers are authoritative for its zone. Essentially, the zone is the group 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 address ought to be retrieved. This way a browser finds out what the A or AAAA record of the domain is so that the latter is mapped to an IP address and the web site content is requested from the correct location, a mail relay server detects which server handles the emails for the domain (MX record) so a message can be delivered to the needed mailbox, and so on. Any change of these sub-records is performed with the help of the company whose name servers are employed, enabling you to keep the website hosting and switch only your email provider for example. Every single domain name has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.