"In Motion" is a DLC expansion designed for PlayStation Move. It's identical to the Sixense expansion released on PC--only you won't need a $100 motion controller to use it. With the Move, you'll get access to a whole new slew of gameplay options, like stretching boxes and rotating objects. You'll also be able to "portal surf."
The DLC will be available for $9.99 ($6.99 for PlayStation Plus members). In addition, Portal 2 will be updated to support PlayStation Move through the entire game. You won't get the fancy "In Motion" controls, as that will break the game. Here, watch what it looks like: