In ophthalmology, mouches volantes are free-floating particles in the eye´s vitreous which impair the eyesight of the person affected. The common English term for these is »eye floaters«.
Such opacities of the vitreous can have different causes; in most cases they are age-related rather than pathological. In bright light conditions, the shadows of these particles are projected onto the light-sensitive retina and thus become visible to the viewer as small rings, dots and strands which appear to »fly« through the visual field, following the eyes´ motions. Eye floaters are widely spread, in itself harmless and not effectively treatable with our current medical means.
This explanation of mouches volantes makes sense to us, even if we are not ophthalmologists, because it refers to our everyday realm of experience accessible with our intellectual mind.