The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Simply, the zone is the group of all records for the domain name, so when you open a URL in a web browser, your personal computer asks the DNS servers around the globe where the domain is hosted and from which servers the DNS records for the domain address must be retrieved. That way a browser finds out what the A or AAAA record of the domain name is so that the latter is mapped to an IP and the website content is required from the proper location, a mail relay server finds out which server takes care of the emails for the domain name (MX record) to ensure a message can be sent to the right mailbox, etc. Any modification of these sub-records is done through the company whose name servers are employed, so you're able to keep the web hosting and switch only your email provider for example. Every domain has at least two NS records - primary and secondary, that start with a prefix like NS or DNS.