Complex sentence: Cone cells, or cones, are one of the two types of photoreceptor cells that are in the retina of the eye which are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light.
Split into simpler sentences: Cone cells, or cones, are one of two types of photoreceptor cells in the retina of the eye. They are responsible for color vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light.