Simply put, a rack focus refers to the act of adjusting which object(s) are in focus during a continues shot. In the video above, you will see that they start with the background in focus, and the foreground out of focus. They then rack focus to the phone, making the foreground in focus, and the background out of focus. This is a prime example of a rack focus. Using this technique is a great was to draw your viewers attention to your subject, or an object on screen.