It comes ready to use out of the box with everything you see above: MaKey MaKey, Alligator Clips, USB Cable.
That's up to you! First, load up a computer program or any webpage (yes that's right, you're surfing the internet to invent).
Let's say you load up a piano. Then, instead of using the computer keyboard buttons to play the piano, you can hook up the MaKey MaKey to something fun, like bananas, and the bananas become your piano keys:
Or let's say you Google for an online "Pacman" game and draw a joystick with a pencil (yes, an actual ordinary pencil):
Then you can play Pacman by touching the drawing with your finger.
Or you could load up facebook or gmail and send a message on a custom-made alphabet soup keyboard: