The NS (Name Server) records of a domain show which DNS servers are authoritative for its zone. Basically, the zone is the collection 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 name is hosted and from which servers the DNS records for the domain ought to be retrieved. With this a 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 website content is required from the right location, a mail relay server finds out which server manages the emails for the domain name (MX record) so a message can be sent to the needed mailbox, and so on. Any change of these sub-records is performed through the company whose name servers are used, permitting you to keep the website hosting and switch only your email provider for instance. Every single domain has no less than 2 NS records - primary and secondary, which start with a prefix like NS or DNS.