2D printing makes things that are flat. 3D printing makes things that have volume. Add in the fourth D, time, and you make things that move. We're not talking about things that can be be moved, but rather, objects that come printed with the capacity to move all by themselves.
MIT’s Self-Assembly Lab has been working on ways to print things in 3D that can change their shape after they've been printed. Routinely printing something as complex as a motor or an actuator is still a ways off, but by mixing different kinds of smart materials, it's possible to get stuff to move on its own, albeit very slowly and (so far) in just one direction.
The process is actually not that complicated, and the smart materials aren't even all that smart: all it takes is a material that acts like a sponge that can be layered inside of a joint during the 3D printing process. When the joint is submerged in water, the sponge material expands and the joint bends. Put a bunch of joints together, and you can get a fairly complex self-assembling object, like this cube:
In this case, water is acting as the energy source that activates the printed smart material, but different materials can use light, heat, vibrations or even sounds. Imagine one day going to IKEA and buying a table made out of this stuff that you can hose down in your front yard to get it to self-assemble. Or, with some sound activated smart material, all you'll have to do is scream at it loud enough and it'll put itself together. Since most of us do this with our IKEA furniture anyway, it seems as though adding in the smart self-assembly bit is the logical and obvious next step.