A pterygium (Greek, meaning wing) is a growth of blood vessels and scar tissue on the surface of the eye. It grows in response to damage from sun, wind, heat, dryness and other conditions that can damage the ocular surface.
If the damage continues, the response itself can cause further damage. The pterygium can become raised, red and irritated. Then the surface dries out, the lid rubs on it when you blink, and it can become sore and inflamed, causing further irritation and growth.