The AppMenu (also referred to as "global menu") is an applet which removes the menu from the application windows and places it on the top panel (see the above screenshot) which results in more vertical screen space. This is especially useful on netbooks, but desktops too can use the extra pixels gained by using this applet.
Even though Unity as been confirmed as default for Ubuntu 11.04 - desktop edition, there were still talks if the desktop version should also use the global menu (AppMenu).
I'm not sure when, but it seems it has been decided that the global menu will be used on Ubuntu 11.04 (desktop) too - this has been confirmed by Jorge Castro @ AskUbuntu:
Yes, the Desktop version of Unity will use the global menu by default. There might be some added intelligence for multi-monitor setups, but I'm not sure and there has been no design as yet.
The global menu will probably work differently on the desktop then in Ubuntu Netbook Edition: we might get a global menu only for maximized windows (there is a discussion going on regarding this on the Ayatana malining list) and, as Jorge Castro points out, the global menu might add some extra features in the desktop edition for multi-monitor setups.
(The above screenshot features the global menu for an maximized window (as well as the window buttons) - note that the top "bar" is not the Rhythmbox titlebar but the top Unity panel)
Of course, if you don't like the global menu (AppMenu), you will be able to remove it (sudo apt-get remove indicator-appmenu appmenu-gtk) and doing this, the menus will return to the old behavior.
And also related to global menu: there is work going on to add support for applications such as Firefox or OpenOffice.