The shortcut the Apple menu reports for the Force Quit menu item is ⌥⌘⎋. (the last character is the one Unicode calls broken circle with northwest arrow.)
What key should I press to invoke the Force Quit menu item?
To be concise: the symbol you're asking about, the circle with the arrow escaping from it (that's the way I remember what it means), is the Apple way of saying 'The ESC Key'.
So the menu item ⌥⌘⎋ is option+command+esc pressed simultaneously.
The Escape key.
I don't know why Apple use this non-standard symbol, but they always have...
Here is a full list of all of the symbols that Apple use.
As an extension to Ian's answer, here's a list of all the keys:
As correctly pointed out by Ian, the ⎋ is the Escape key located on the upper left side of your keyboard.
It is the escape key (esc) it is on the top right or left corner of the keyboard
You can find a list of all the keyboard icons, terminology, and HTML entities over here on Meta . That icon ⎋ is the Escape key. You can find it in the very top left-hand corner of your keyboard.