This will allow you to set custom icons to use on each individual forum. You can specify a path to your forum's custom icon by going to the ACP => Forum Management => Edit Forum.
When installed, it will alter the forums table to add a column called myforumicon, which will be used for the custom icon of your forum.
When activated, modifications to your forumbit_depth2_forum template will be made. This is done to allow the plugin to generate an icon for the forum and hide the default icon. When an icon is not set, the default will be used.