Retinal veins drain blood from the retina. They can become blocked due to compression from adjacent arteries or at the point of exit of the vein from the eye. This can lead to a build up of pressure in the veins, resulting in leakage of fluid at the macula. The macula is the central part of the retina; most critical for detailed vision.
The eye may also react to this insult by developing new blood vessels in the retina. These can lead to further visually significant complications.