in-home streaming service is available to all users starting today – for now, the hosting computer must be a Windows PC, but Valve says SteamOS, Linux and Mac OS X hosting is coming soon. Users with multiple computers on the same network are now able to play PC games on lower-end computers, and play PC games on Mac OS X, SteamOS or Linux.
Yes, Dark Souls 2 on your Macbook Air.
To stream, users must log into Steam on a Windows PC (for now), and then log into Steam on another computer on the same network. Then check out your Steam library and start streaming some games. Once a game is installed on one computer, there's no need to install it on the other streaming computers. As Steam describes it:
"When you play a game using In-Home Streaming, video and audio are sent through your home network from your high-end gaming PC to another device in your home. From here, your keyboard, mouse, and controller input is sent back to the remote computer."
Steam's breakdown of the in-home streaming service can be found here.