The cornea is the clear window on the very front of the eye through which the pupil and the colored iris is visible. The cornea has two major functions: focus the light rays that come into the eye onto the retina and protection of the eye.
To focus the light and make a clear image on the retina, the cornea has to be exquisitely clear and without distortion. This happens because the cornea is constantly moistened and polished by the lids making its anterior surface as smooth as glass. The backside of the cornea has a special layer of pump cells (endothelium) that allow nutrition and clearing of the cornea without blood vessels that might blur vision.
Protection is provided by the cornea by its very numerous and sensitive nerve endings on its surface. This prevents injury because it leads to significant pain if scratched or injured. The cornea is also rigid without being brittle so that it holds its shape even under extreme conditions. This rigidity keeps the image in focus.
If disease or injury destroys any of these functions then the cornea will not be clear causing the vision to be blurred.