The addition is called "The Steam Inventory Service" and it only exists in beta form thus far (and therefore only to users who opt-in to the beta version of Steam). Here's how Valve describes it:
"The Steam Inventory Service is a set of new Steamworks APIs and tools that allow a game to enable persistent items that have been purchase or unlocked by individual users without having to run special servers to keep track of these user's inventory."
Snoozefest, we know. The long and short is that game creators can now add unlockable item systems to their games, and those unlocked items can be "marked as tradable through Steam or sellable via the Steam Marketplace." Which is to say: the economy of items that's existed for so long through Valve's own games is now opening to the wider Steam world. And that's a good thing! Well, kind of.
On one side, it means that game developers are potentially able to add more value to their games -- play our game for 50 hours and your in-game character gets a special sash noting your dedication! On the other side, there's of course potential for abuse: game developers could start building their games with economies like this in mind, and Valve's unlikely to police those games from its service.
It's all very early days, naturally -- the system's still in beta and we've yet to hear any devs signing on just yet. A year from now, though? We could be talking about the next $50 million Valve's paid out to users who are creating and selling content on a vast swath of games.