The vitreous (the gel-like fluid inside your eye) is tightly attached to the retina which is then attached to the optic nerve; the channel through which images travel to your brain. Sometimes the vitreous retreats, pulling the retina away from its place at the back of the eye. Other times, a hard impact can cause the retina to detach. Regardless of how it happens, a retinal detachment is a serious issue.
If you start experiencing waves of flashes, a downward shower of floaters, or the appearance of a black curtain falling over your vision, you need to seek medical help immediately. A detached retina can usually be repaired or reattached, but time is of the essence. The sooner you see a doctor, the more likely you are to regain your vision.