It is a clear gel, which fills in the space between the retina and the lens. If it becomne foggy or if it was glazed over it would be harder to send the picture that the eye sees to the optic nerve,...
To give structure to the eyeball, and provide a clear medium for the light to pass through, so that it lands on the photoreceptor cells in the retina, the back of the eye.
The vitreous humor is a jelly like liquid that fills most of the eye (from the lens back). As we age it changes from a gel to a liquid and gradually shrinks separating from the retina. See related...