Originally Posted by pixellany
I think he means "ctrl-alt-escape"
But I would assume this depends on what terminal you are using.....
It's Control-<letter a> Escape.
It's not related to any given terminal, it's related to GNU Screen (sorry if I wasn't clear enough before), a terminal multiplexer that can do lots of things, amongst them, it allows you to copy/paste without using a mouse.
A random web I just googled with some basic info about how to use Screen:
Of course, first you need to install it, then you can use it on any terminal emulator, or even plain command line. It might be a bit painful at first, but it's once of these things that once you learn you can't live without it.