The eyeball is filled with a jelly called the vitreous. At birth the vitreous is attached to the retina, however over time gradually it separates and pulls away. This is referred to as Vitreous Detachment or Posterior Vitreous Detachment (PVD).
Vitreous detachments are very common and are a normal part of the ageing process. They do not cause blurring, however they can lead to more floaters being visible. There may be no symptoms, or you may see flashing lights and floaters.
On the whole the condition is harmless and requires no treatment. However occasionally the jelly pulling away can tear the retina, which does require prompt treatment to avoid a retinal detachment. It is therefore strongly advised to have your eyes checked if you experience a sudden onset of persistent flashing lights, lots of new floaters or a curtain or shadow in your field of vision.