It means the sun & moon came second, after spiritual creations came first. Unfortunately, the author is confused, because the sun is a spiritual creation. It's just a ball of light. Pure energy. In Christianity, God's first creation was light (or more generally: energy), which is represented by the sun, and that's why we say the first day is Sunday (sun day). After that, came solid matter, like the Moon and the Earth, which is why our second day is Monday (moon day).