The Toolbox is the heart of the GIMP. It contains the most commonly used controls, and the highest-level menus. Two of the menus, File and Help, can also be accessed from the menubars located above each image display, although their contents are somewhat different. The Toolbox is the only place to access the Xtns (Extensions) menu.
You may find others entries in this menu-bar. They don't belong to GIMP standard data but have been added by some plug-in.