Life is full of movement and activity. As you age, bits and pieces of your retinas break loose and float in the inner liquid of the eye (the vitreous.). While these floaters are irratating, they are harmless. They tend to be noticed when you are looking at white backgrounds such as walls, the sky and while reading.
However, some flashes and floaters can let you know something is happening to your retina (the inner back tissue of the eyeball). You could be experiencing vitreous detachments or a detached retina, which must be diagnosed and treated.