what does this quote mean to you?
Maybe it means that if you haven't suffered "darkness" and sadness and hurt and pain and all that, you can't expect dreams and opportunities in the future. You've got to have some experience.
Excluding the obvious (you dream when you sleep at night in the dark), I would say that it's like you can't have happiness without sadness. If there were no darkness (no sadness, trouble, pain, toil, death, misfortune, etc) in life, why would there be need to dream? You dream of things in or for your life that would make it better, but if everything is perfect, there's nothing to dream about. Nothing can make perfect better.
That if there is anything 'dark' or bad, you can't dream up anything better