The module retrieves files in /media from an origin server.
Imagine you have a fresh local development environment with the Magento code checked out. You retrieved the database but you don't have any of the media assets and your store frontend just looks incomplete. You could grab a huge archive of the media folder from the production environment but no one really wants to download tens of gigabytes just to work on a few catalog pages.
This modules implements some plugins and observers that downloads images of categories, products and CMS blocks/pages from a configurable origin server similar to a CDN the first time you load an entity from the database. This means you can forget about the media folder and just browse the frontend as images are downloaded and saved transparently.
In the module's configuration you can configure a base URL, the domain where your production/staging Magento instance is located from which to picked the database. In the database the relative paths of categories and product images are stored. Once those entities are loaded the module simply checks if their images are already in media/catalog. If not it uses the base URL, appends the relative image path from the database and downloads the files from origin server. This slows down page generation the first time you access a page but improves pretty quickly.
For other assets in the media folder we make use of another mechanism: Maybe you recognized Magento is shipped with a get.php file in the Magento root. It was intended to retrieve image data from a database, save it on the host's filesystem and then deliver it. Well, our assets are located at a different web server but beside the retrieval of the asset data the rest is pretty similar. The get.php is called via a mod_rewrite rule in the media/.htaccess or equivalent rules in your nginx configuration. The process is triggered every time a file in /media is not found so it is only triggered the first time. The MediaStorageSync module downloads the file, saves it and the get.php delivers the file. On the second load the web service can directly deliver the static asset.
Manually copy the files in the app folder to your Magento app folder and clear the cache.
Find the modules configuration in the PHOENIX MEDIA section of your Magento configuration.
Enable: Enable or disable the functionality
URL: Configure the source URL where to retrieve the images (e.g. "https://magento.com/")
optionally configure credentials for BasicAuth.