"This section is intended for items looking to gather feedback from the potential customers and begin building a community," the Concepts description reads. "If you like projects here, feel free to rate them, favorite them, and leave feedback for the developer. The voting here serves only to give the developer data and reactions and doesn't work toward getting the game distributed on Steam."
Concepts offers a compromise between the completely open system that Greenlight launched with (and quickly amended), and the $100 fee now required for hosting a game on Greenlight. It allows developers to see if their games have a chance, to generate a following and resources, before taking them to Greenlight or elsewhere.
Concepts joins the "Software" tab recently added to Greenlight. As of publishing, there are only 15 games on Concepts, so if you have a potential indie hit, maybe get it up there now, while the getting's good.