With Rhythmbox's default configuration, there's a "Tools" element in the AppMenu with an arrow next to it, so you'd expect it to expand the menu to show some options when clicked (as "View" does). However it doesn't show or do anything at all when clicked, doesn't have any effect or provide any feedback. If and only if you activate the Python Console plugin, the "Tools" element does expand the menu when clicked and show an option. So I'm guessing it's a container for plugins options. Needless to say, it shouldn't show up in the AppMenu if there's no plugin activated that makes use of it.