A commonly used technique in both film and television to shift focus from a foreground object to a background object by turning the the focal ring on a lens.
Examples of rack focus can be seen in almost any movie or tv show when the focus is shifted from one object--mostly people--to another. It is most often used dramatically.
When a director forgets what he is shooting and films breasts up close.
"The movie is still in editing becuase the second scene was rack focused."
When a cinematographer turns the focal ring on his lens to shift focus from a foreground object to a background object.
A rack focus is really a cheap, metaphorical device for the unimaginative to link two objects/people in a frame.