The vitreous lies against the retina in the normal eye. The eye is like a small ball, the size of a table-tennis ball.
Light enters the eye, and then passes through to fall on the retina.
The retina turns the light into electrical signals, which are then sent to the brain. As you get older the vitreous may shrink away from the retina. This may happen earlier if you are short sighted or have injured your eye. This process is called a posterior vitreous detachment. The vitreous may shrink in different ways.