The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. In simple terms, the zone is the collection of all records for the domain name, so when you open a URL within a browser, your laptop or computer asks the DNS servers worldwide where the domain address is hosted and from which servers the DNS records for the domain must be retrieved. In 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 website content is required from the right location, a mail relay server finds out which server deals with the e-mails for the domain name (MX record) so that a message can be sent to the needed mailbox, and so forth. Any change of these sub-records is done using the company whose name servers are used, enabling you to keep the website hosting and switch only your email provider for example. Every single domain name has a minimum of 2 NS records - primary and secondary, which start with a prefix like NS or DNS.