Within the confines of Valve's HQ in Washington, no doubt many projects are on the boil -- or frying or blanching -- whatever it does to create its unique type of magic. Some internet sleuthing may have uncovered one such enterprise, though all we know is that it's called "Trading Card".
An image offering beta access to Trading Card can be found on Steam's content distribution network (FYI, it's the same as the one above). SteamDB has slightly more information, but nothing that reveals its purpose.
Given the name, you might posit that it's related to some sort of trading or collectible card game, though you'd think it'd be branded as an actual game, rather than a concept. It'd be like Wizards of the Coast calling Magic: The Gathering "Richard Garfield's Trading Card Game".
What's more likely is that it's a new Steam feature or service. It could be tied to Valve's plans to make Steam a more open platform, providing developers and publishers with the tools necessary to sell games without having to go through the rigmarole and largely flawed system that is Greenlight. The "trading card" in this case is a virtual credential of sorts, making one a verified seller.
But that's just a guess. Maybe it is a weird card game where people trade Team Fortress 2 hats for a legendary Gabe Newell card with 20/20 stats and a tap ability that allows you to create an "Award Winning Game"... as long as you don't mind waiting three million turns to untap.