Valve announced the Linux-based SteamOS in 2013, just prior to the reveal of the vaguely console-like Steam Machine PCs. It was a big, bold move that ultimately petered out: Valve ditched the Steam Machines section of its website in April, aalthough you can still hit it directly if you know the URL.
SteamOS is still alive and kicking, though—in fact, the 2.164 update was pushed just a couple of weeks ago—and now it appears that Valve is aiming to dramatically broaden its capabilities. Members of the r/linux-gaming subreddit recently discovered text strings referring to, among other things, "compatibility tools" that would enable games designed for Windows—which is to say, pretty much all of them—to run on Linux. The functionality will apparently be similar to that of the well-aged Wine, but with the added muscle of official backing and support from Valve.
Other uncovered strings suggest that the compatibility wrapper will work better with some games than with others, and that compatibility will be (or could be) a store-listed feature: One says that Steam Play can be used "to test games in your library that have not been verified with a supported compatibility tool," while another says users can attempt to use the tools to run unverified games, but that doing so can cause issues "including crashes and breaking save games."
As Ars Technica notes, the discovery of the text strings is a long way from an official rollout, and should be treated as such. There's no indication of how the system will work, or even what exactly it's for, and while it's been a few years since testing took place, gaming performance under SteamOS was found to lag behind Windows 10, and that's a big problem if you're trying to convince people to make the switch.
The ability to run Windows-based games in Linux would be a major boon, and if anyone can make it work, it's Valve. But it'll probably be a good while yet before the launch party invitations start to go out.