You can bind an object in your hierarchy to any other object (often an object not in the same hierarchy). This other object is called the follow object.
The bound object tries to match the position and orientation of its pivot point to the position and orientation of the follow object's pivot point.
For example, you might want to animate a figure that always points to another object. Bind the hand of the figure to the other object. Turn IK on and as you move the object the hand and arm of the figure move to point at it. See Position/Orientation?Bind to Follow Object (HD Solver).