In filmmaking, a rack focus (or pull focus) is when the filmmaker changes focus within a single shot in order to draw the viewer's attention to something else.
The general concept is that there are two objects onscreen: one very near to the camera, and one further off in the distance, but still visible. The camera will be focused on one of the objects, and then will shift focus (by shortening or lengthening the focal length of the camera) to the other object.
For amateur filmmakers, this often requires a bit more practice, since autofocus abilities on most digital video cameras are rather lackluster and shift focus too frequently. Instead, use manual focus and a speedy zoom to create cognitive dissonance or additional resonance among your viewers.
You can spot rack focus in a number of films, including The Fugitive, City of Angels, Rob Roy, and, of course, Touch of Evil.