This is described in the NEWS file for Emacs 24.4:
* Emacs now supports menus on text-mode terminals.
If the terminal supports a mouse, clicking on the menu bar, or on
sensitive portions of the mode line or header line, will drop down the
menu defined at that position. Likewise, clicking C-mouse-2 or
C-mouse-2 or C-mouse-3 on the text area will pop up the menus defined
for those locations.
If the text terminal does not support a mouse, you can activate the
first menu-bar menu by typing F10, which invokes `menu-bar-open'.
If you want the previous behavior, whereby F10 invoked `tmm-menubar',
customize the option `tty-menu-open-use-tmm' to a non-nil value.
(Typing M-` always invokes `tmm-menubar', even if `tty-menu-open-use-tmm'
is nil.)
Looks like a nice feature.