I imagine these will just be an extension of the Weekly Toy Box Challenges already there, except with greater dedicated team involvement.
Every week/two weeks/whatever, they will release official Toy Box Tales levels, these will be complete full toy box games with a narrative, as they said, but at the end of each box it will give the player an objective: they'll need to make their own level to fill in the gap between this Toy Box Tale and the next one. The best ones will be promoted on the Toy Box Downloads page as the current challenges are.