What does a key sequence beginning with A- mean in emacs?

Question

I'm playing with coffee-mode for editing coffee-script, but I'm a total emacs newb.

In coffee-mode there are key bindings like "A-r" for compiling a buffer and "A-M-r" (I assume the "M" is meta) to open a coffeescript repl. However I can't find which key "A" refers to (it's really hard to google for A-).

Answer 1

I figured out the problem. "A-r" apparently is the command key and r on a mac!