Read about it here
What this means is that stress is taken off your CPU and put onto your GPU when playing back Flash video (SD or HD) from within a web browser. This should be really helpful for those running weaker CPUs, like a ~2.0GHz mobile Core 2 Duo.FLAWLESS FLASH VIDEO
Put an end to stuttering online videos and enjoy smoother, full screen SD and HD content with an NVIDIA® graphics processor. Adobe Flash Player 10.1 beta is here, with full GPU-acceleration for NVIDIA® ION™ and NVIDIA® GeForce®-powered PCs; and support for NVIDIA® Tegra®-powered devices coming soon. Visit these HD Gallery pages from your favorite websites to view GPU-accelerated Adobe Flash 10.1 content. The full production release of Adobe Flash Player 10.1 will be available in 1H 2010.
You can get the Flash v10.1 beta from here. The latest Nvidia drivers can be downloaded from here.
Nvidia have a list of supported GPUs here. I have no idea of whether or not ATI Radeon drivers support this feature.