The MENU key is referenced within Haiku's documentation, but sadly, this mediocre, and yet powerful, key isn't explained very well. It's not the super, meta, or alt/option key, but rather is another key which is disappearing from modern-day mobile (and even some desktop) keyboards.

This reference refers to the Context menu key that existed on legacy PC keyboards, usually crunched between the lower right modifier keys (Alt, Ctrl) or by the special/navigation keys (page up/down, home, end, etc.) allows you to open the Haiku (i.e. Leaf) menu, so that if you do not have a mouse or other pointing device available, you can still open the menu and access core system functions, like Preferences and the ability to restart, easily.