Simply put, diabetics have too much sugar (glucose) in their bloodstream, which can lead to damage of the small blood vessels in the body, most notably the eyes, the kidneys, and the feet. In the back of the eye (the retina), early damage can be seen as small hemorrhages (spots of blood) which can then lead to larger hemorrhages, leaking of blood vessels, swelling inside the eye, and more if better blood sugar control is not established. If changes are seen on the retina, it is very likely that there is damage to other small blood vessels elsewhere in the body.
It is recommended that all diabetics have annual eye exams with pupil dilation. Diabetes is the #1 cause of blindness in the U.S. and early detection and treatment is better than later.