StageVideo is hardware-accelerated video playback. What does that mean? It means that Flash puts more of the burden on your graphics card (or GPU), reducing the burden on the CPU.
The result? Massive performance savings. Of course, it depends on what you are trying to render on the screen, but in my talk I show Flash playing an HD video with some simple vector graphics rendering on top — without StageVideo I'm at 75% processor usage, and with StageVideo, I was running at 10% processor usage. WOW! Right?
What does that actually mean in practice? It means that you can deliver super high-res HD video through the Flash Player without dropping frames to more and more devices. I mean, the video quality that Flash Player can now support is pretty intense and amazing.