Rack Focus can either be from out of focus completely to focus on a particular subject or from one subject to another. It generally uses a narrow depth of field and a follow focus may be used to help reach the focus points exactly and in a controlled manner.
The technique is often used either to introduce a subject from a dream or stunned like state (for example, if someone is waking up or in an accident, their pov might start out with a rack focus from being completely out of focus). It is also very frequently used to shift the viewer's focus from one subject to another in a dramatic way. In the more artistic sense, it can also be used to simply explore the depth of the scene by moving the viewer through the space.