WHAT

This is a plugin of CKEDITOR

It check your content of EDITOR, find each <img src="url" ... >, then request your server

The server download the image the "url" specfied, and return a new "URL", which is the url of downloaded image in your server

Then the plugin replace the "url" with the new "URL", so your content no longer do not depend on the external resource.

FOR　WHAT

make your image in the EDITOR's content link to your site, do not depend on external resource