To demonstrate how this works, drag the slider to change the Focus Distance between 4m, 7m, and 10m. Adjusting the Focus Distance adjusts the Focal Plane (purple dotted line) in the scene, indicating where the in-focus region of the shot is. The blue reference point in the diagram uses the Character as the focal subject here, so as the camera shifts shot focus (black/gray area above) to different parts of the scene, objects in the foreground and background will shift too.
Starting with 4m, the Police car in the foreground is in sharp focus and the character is out of focus in the background. The blue reference in the diagram indicates this with its focal point intersection in front of the filmback. At 7m, the character is completely in focus with background and foreground objects out of focus. At 10m, the construction barriers are in sharp focus and the character and Police car are out of focus in the foreground. The blue reference in the diagram indicates this since the intersection point extends beyond the filmback.
Through all these Focus Distance changes, the DOF effect increases or decreases depending on an object being in the foreground or background.
Note that only the Focus Distance is being changed here; the F-stop and Focal Length remain constant at f/1.4 and 75mm, respectively.