The concept is that you can algorithmically figure out what the person entered as a passcode with high accuracy for numeric codes and impressive accuracy for alphanumeric passcodes. The question is exactly how much of a threat does this thing represent, since you need a fairly clear view of the device that's being used.
http://www.wired.com/2014/06/google-glass-snoopers-can-steal-your-passcode-with-a-glance/