Protocol Relative URLs makes all file URLs generated by Drupal to be protocol relative.
What are protocol relative URLs?
Protocol relative URLs look like:
//example.com/foo/bar. Dropping the protocol from the URL allows your web browser to assign the current protocol to the URL. So, if you are visiting the site in HTTPS mode, links will point to HTTPS, and if you are visiting the site in HTTP mode, links will point to HTTP.
* Note: protocol relative URLs will cause IE7/8 to download stylesheets twice.