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 range of all records for the domain, so when you open a URL within a browser, your PC 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. That way a 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 proper location, a mail relay server discovers which server handles the e-mails for the domain name (MX record) so that a message can be delivered to the right mailbox, etc. Any change of these sub-records is conducted through the company whose name servers are employed, allowing you to keep the web hosting and switch only your email provider for instance. Each domain has no less than two NS records - primary and secondary, which start with a prefix such as NS or DNS.