Essentially, it is that instead of just the key turning the tumblers in the ignition, there is an electronic signal also passed from the key to the car which allows the car to be started.
Since every car I have seen in the past 10 years or so has had this, I would think that the Acadia key also has a chip in it to perform this function.
What I was a little surprised by is that you have to put the key in the ignition at all. I would have thought that the Acadia being totally new would have been using an RFID off the key fob to identify the driver.
I would think that due to the remote start function (way cool), that's why you have to put the key in. That way nobody can drive off in it as you are on your way to it after remote starting it from the balconey of your third story condo.
Actually, a lot of cars have remote start and are keyless. Since for remote start to work, the first step is to lock the door-- later you have to unlock it to get in, and then you push a button (or on the CX-9, you turn a small handle in the normal key area) and as long as the key fob is within the confines of the vehicle, it gives the OK to keep running.
You could even do simple biometrics thumbreader to drive the car, but I don't know if anyone does that (my laptop does though!)