The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. In simple terms, the zone is the group of all records for the domain name, so when you open a URL within a browser, your personal computer asks the DNS servers around the globe where the domain name is hosted and from which servers the DNS records for the domain must be retrieved. This way 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 address and the web site content is requested from the correct location, a mail relay server discovers which server handles the emails for the domain address (MX record) so a message can be delivered to the right mailbox, etc. Any change of these sub-records is done with the help of the company whose name servers are used, so you're able to keep the website hosting and switch only your email provider for example. Every single domain address has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.