explain what posterior vitreous detachment is?

Answer 1

Here is a whole article about it - pretty readable, not just medical jargon!
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A posterior vitreous detachment (PVD) is an ocular condition in which there is a separation of the vitreous humor from the retina.

The vitreous humor fills the eye behind the lens. At birth it is attached to the retina. Over time the vitreous changes, shrinking and developing pockets of liquefaction, similar to the way a gelatin dessert shrinks, or detaches, from the edge of a pan over time. At some stage the vitreous may peel away from the retina. This can be, and usually is, a sudden event. When this occurs there is a characteristic pattern of symptoms: