The retina lies in the back of the eye and is a multi-layered tissue which detects visual images and transmits these to the brain. In front of the retina lies the vitreous humor. The vitreous is the jelly-like material that fills the large central cavity of the eye. It is composed primarily of water, but it is also made up of proteins and other substances which are more fibrous. The water and fibrous together give the vitreous the consistency of gelatin.
The vitreous is normally connected to the retina. During aging, the watery portion of the vitreous separates from the fibrous portions. As this occurs, the fibrous elements contact and can pull the vitreous away from the retina. This is called Posterior Vitreous Detachment. This contraction on the retina is responsible for the characteristic flashes that often accompany the Posterior Vitreous Detachment. The floaters are frequently caused by the fibrous elements changing position during the Posterior Vitreous Detachment. They can also caused by pieces of the retina being dislodged as the vitreous contacts. Besides aging, flashes and floaters are also associated with nearsightedness and injuries to the eye.
Floaters, if present since a long time or increasing in number very gradually, are harmless. They may be annoying but do not cause any problems. However, if a new floater comes on abruptly or if there is a sudden increase in the number of floaters, then you must be examined immediately to rule out a retinal tear which could lead to a retinal detachment.