Mixed reality here means VR goggles like Oculus. You're looking at a screen inside some closed-to-the-outside-world goggles. What makes it "mixed" reality as opposed to VR is that the goggles may have an outward facing camera used such that what you see on the screen combines this camera view with some VR aspects. This isn't something that you are going to be walking down the road wearing though - it's basically a 3-D gaming headset in terms of looks.

The AR product that Apple have apparently put on hold is more like Google glass - where fundamentally is it a pair of glasses that you can see through, but with the AR component somehow projected into your field of view too. So you're basically looking at the real world with pop-up information, virtual labels on things, maybe Pokemon skooting around too.