The Static Content plugin allows you to replace the URL to your images or CSS-files with another URL. While you have the responsibility to make sure that this URL is actually serving the same images and CSS files, this allows for quick integration of static webservers or CDNs in Joomla.
To a minimum you need to configure Unsecure URL for the plugin to do anything useful. This URL has to start with http:// and should host an environment that mirrors your normal Joomla! environment. For instance, if your Joomla! template-images are located here:
Then your static server should contain the same folder templates/mytemplate/images.
When entering a value under Secure URL, make sure this is a SSL-based URL. Also consider using a trusted SSL-certificate for both the Joomla! site as the static server (or CDN).
Other options allow you to determine which resources should be moved to the static server:
When loading CSS files from the static server, note that you also might need to copy the neccessary image-files. Because the CSS-files will be served from the same directory-structure as within Joomla!, relative path-links will still work.