The vitreous is a gel like substance which fills the center of the eye. With age or injury it liquifies and pulls away from the retina. Cells, membranes, and protein like strands float in the vitreous and appear to the person like cobwebs, spots, or lines. They are called floaters.
Retinal tears can occur and as a result a retinal detachment can occur.
The flashes and floaters are troublesome but are good warning signs that a full eye exam is necessary to rule out damage to the retina.