The NS (Name Server) records of a domain point out which DNS servers are authoritative for its zone. Essentially, the zone is the group of all records for the domain name, so when you open a URL within an Internet browser, your laptop or computer asks the DNS servers world-wide where the domain is hosted and from which servers the DNS records for the domain address should 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 right location, a mail relay server finds out which server takes care of the emails for the domain name (MX record) to ensure that a message can be delivered to the appropriate mailbox, etc. Any change of these sub-records is performed using the company whose name servers are employed, allowing you to keep the web hosting and switch only your email provider for example. Every domain name has a minimum of two NS records - primary and secondary, that start with a prefix like NS or DNS.