Basically it's "Minecraft" with pre-fab building pieces for you to recreate your own version of the Magic Kingdom. Check out the trailer below:
From the release:
Utilizing the logic editor tools and accessible tutorials, players can build their own environments from scratch or use pre-built templates. Some examples of the creative worlds that can be built in Toy Box can be seen in the new video, including a floating race track, a massive version of Cinderella’s castle complete with rings of fire and a world where a pirate’s ship is sitting pretty as the centerpiece.
Players can let their imaginations run wild by creating adventures and mini-games within these customized environments. They can create obstacle courses, sports games or even re-create classic arcade games by using a variety of characters such as Mr. Incredible and Jack Sparrow, and vehicles including the Tron recognizer while fending off opponents with a variety of combat tools.
Seems pretty cool and I expect to see some truly inspiring creations on youtube in the following months after its release.