Ask an Eye Doctor and Get an Answer ASAP
How long have you noticed it.
Is it stationary or does it move when you move your eyes?
Any flashes of light?
Are you nearsighted or farsighted,
The eyeball is a cavity and is filled with a gel like substance called the vitreous.
As the vitreous ages it breaks off into threads.When it is a new occurence this thread is still attached to the lining of the back of the eye , the retina, such that each time you look in a certain direction that threadlike line will move.This is commonly known as a posterior vitreal detachment.
When it is still attached to the retina you may also get flashes of light.
The main thing is to follow-up with your eye care doctor to have your eyes dilated so he can better evaluate it.
Any other questions feel free to ask.
Otherwise please do rate my answer.