Video and written answer:
Qualified optometrist Nigel Best BSc (Hons) MCOptom FBCLA FAAO answers this question:

This is a condition which affects some diabetics after a number of years, depending on how well their insulin levels are controlled. In 'non-proliferative' retinopathy the blood vessels in the retina dilate, altering the blood flow in the retina. This condition usually causes no symptoms and is not sight-threatening, however it needs to be monitored carefully because it may signal future damage to the back of the eye. In 'proliferative' retinopathy, which can develop from the non-proliferative stage, new blood vessels form in the eye. These vessels are extremely weak and also in the wrong place. As a result the vessels can break and leak blood. This causes both scar tissue to develop and the fluid in the eye becomes cloudy. Occasionally, retinal detachment can occur.

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