"Fixation" means that the point F is aligned so that its image falls on the "fovea" at the center of the retina where vision is sharpest. As you move the various controls, notice that the red lines from F to the back of the eye include a diameter of the cross section of the eye. In particular, move F and watch the eyes.

The point D represents another object that the viewer also sees, but it is not her primary visual attention.

The scale is very small, with the fixate point 10 from the base point. This scale exaggerates the angles to help see what they measure at normal viewing distances.