What is external page caching in drupal?

@dalton_moen 

External page caching in Drupal refers to the practice of caching rendered HTML content outside of the Drupal application, typically at the web server or reverse proxy level. This is done to improve performance and reduce server load by serving cached pages directly to users without having to execute PHP code or query the database for each request.

External page caching can be implemented using various caching solutions, such as Varnish, Nginx FastCGI Cache, or CDN services. By caching pages externally, Drupal can offload the task of serving cached content to a cache layer closer to the user, resulting in faster page load times and improved scalability.