There is a sack containing vitreous jelly inside they eye.

This sack is attached to the inside of the eye - the Retina.

If the eye is subjected to trauma, the sack can pull against the Retina.

If the trauma is sufficient, the sack may tear a section of the Retina.

You may experience flashing lights in your peripheral vision.

Sometimes the Retina may pull away from the eye wall ...

... and fluid flow in to fill the space - a Detached Retina.

Cells may also enter the vitreous jelly and cause an increase in Floaters.