The extension will simply display an icon that allows you to access The Old Reader (http://theoldreader.com/) directly from your toolbar.
A badge will indicate the amount of unread messages that await your attention.
Packaged extension can be installed from http://theoldreader.com/safari/The%20Old%20Reader.safariextz
Unread message badge can only be refreshed in the background if your cookies and website data preferences are either "Allow from websites I visit" or "Always allow". It will not be able to refresh on "Allow from current website only" or "Always block" unless you are logged to theoldreader.com AND have a window or tab currently displaying it.