The idea behind URLsum is to assign a specific keyphrase to a URL. Such registered sites are afterwards accessible for anybody of the world via these phrases without the knowledge of their real addresses. The keyphrase is MD5 hashed and therefore not human readable anymore. After installing and activating the plugin (left click on the icon in the status bar) you can access registered sites by simply typing the used keyphrase into the address field of your browser. Right click on the icon gives you the opportunity to register new keyphrase - URL assignments.
Hash and URL will be stored on the urlsum.com servers and so are accessible for anybody with any browser or urlsum plugin!