To jiggle is to move very quickly back and forth or up and down. If you ride in a car over bumpy roads holding a bowl of Jello on your lap, you can watch it jiggle until you reach your destination.
You might feel your body jiggle when you ride on a galloping horse, and you can also jiggle something, like when you accidentally jiggle a chess board and send the pieces flying. The action of moving in this way is a jiggle. Jiggle comes from jig, "lively dance," and is probably an alteration of the earlier joggle, "move with jerks or small bobs."