A dummy object is a non-rendering object that you use as an animation helper. The primary use of the dummy helper object is to assist you in creating complex motions and building complex hierarchies. Because dummies are invisible in the rendered scene, they are an excellent choice for offset joints, connectors between objects, and handles for manipulating complex hierarchies.
Breaking complex motions into simple components often makes it easier to go back and edit your animations. For example, consider animating a bouncing ball moving around your scene. You could animate the ball by properly positioning it throughout the scene on many frames. The drawback is that it would be very difficult for you to go back and adjust the height of the bounce or the path the ball takes through the scene. You would have to edit the motion of the ball on many frames to make even a simple change.
Using a dummy object easily solves this problem by breaking the motion into two simple components. One component is the bounce of the ball. The other is the path through the scene.