See also Using the Depth of Field filter.
Specifies the width (in centimeters) for the area of sharpest focus in the 3D View. Regions outside the Depth of Field are blurred based on the Blur Amount value.
Specifies the distance (in centimeters) from the viewing camera to the center of the Depth of Field area. The Focus Distance is connected to the camera. That is, as you dolly the camera back and forth, the Depth of Field also moves in relation to the object.
Specifies the amount of blur outside the Depth of Field area when simulating the depth of field effect. Lower values blur areas outside the Depth of Field to a lesser degree than higher values.