Apple - What is this symbol? ⎋ (it is listed as menu shortcut)

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:

Apple Special Key Symbols

As correctly pointed out by Ian, the ⎋ is the Escape key located on the upper left side of your keyboard.