they are small objects, debris, that float in the vitreous body, the gel-like substance that fills the interior of the eyeball...they can and do move and when they are in between the lens and the retina, they become visible as a dark spot...this one is right in line with where my focal point is for reading, and I have to blink and look slightly off center to read.
Exercises call for moving my head up and down, left and right, and around while moving my eyes/looking in the opposite directions...I get a little dizzy if I do this too vigorously...