The experiment, aptly dubbed "Play Next," uses Steam's recommendation algorithms to suggest three unplayed games from your library. They could be games you bought in a sale years ago, or a title you just picked up a few days ago -- as long as it's never been launched, it's fair game (no pun intended).
Of course, Steam's algorithms are a bit more sophisticated than "if it's unplayed, suggest it." Play Next takes into account other games you've played in the past before giving its recommendations, so hopefully, you won't receive too many suggestions for games you got in a Humble Bundle, but never had any intention of trying.
As we said, Play Next selects three games at a time, but if you don't care for any of the recommendations, you can just hit the "next picks" option to generate three more. Naturally, there's a limit to how many times you can do this -- if your Steam library only has a few dozen games, there's a good chance you've probably played most of them at least once.
Still, Play Next is a neat experimental feature, especially for those (like myself) whose Steam libraries have become a bit too difficult to manage. You can try it now by visiting the Steam Labs tab on the Steam storefront.