As its name suggests, this initiative will allow users to stream and play any game from one PC to another as long as both are on the same network. Valve hopes that this will speed adoption of Steam Machines by alleviating the hassle of moving your current favorite games from your PC to the new device. If nothing else, being able to stream games from an extant computer to another gadget will save users the hassle of having to re-download hundreds of gigabytes of video game data.
Invites to the beta test have been issued at random to members of the Steam Homestream community. If you have yet to receive an invitation, don't fret: Valve will be adding more participants to the test as time rolls on. For more details on Steam in-home streaming, have a look at Valve's official FAQ.