For example, a car is a moving object, and the gap between the car and the car in front is active space as the space is constantly moving as well. If a collision was to occur with the car in front, the car in the back needs active space to react and take maneuvers to avoid hitting the car.
True You can create a dreamy look to a photograph by using a large aperture and then focusing either on an unimportant piece of the photograph or by focusing well in front or behind your main subject.
