Everyone sees things that aren't real sometimes. Look at a bright white wall or stare up at a blue cloudless sky and you will probably be able to spot some odd black wispy blobs floating across your vision. These are perfectly harmless 'floaters' caused by tiny, near-transparent fragments of old tissue that have broken off from the back of the eye and are swimming around in the fluid inside the eyeball.
A sudden 'shower' of lots of floaters may be the first sign of a 'retinal detachment' or a 'retinal tear', which means that the delicate light-sensitive sheet of tissue at the back of the eye is starting to peel away.
Getting help quickly means the eye doctor can fix the problem by 'gluing' the retina back in place with a surgical procedure.
Source Benenden Health
