노을 is the beautiful and colorful sky at sunset. At sunset, we can see the sky becomes colorful and the color spreads out broadly. In this sentence, 노을 is like a person. Therefore, 노을아, 노을아 is calling the name. That sentence means, 노을, 노을, pull down the sky! (In other words, beautiful sky spread out broadly.)

I think you've watched the drama, 동이(Dong Yi).^^ Please visit the following link, you can get more than that. http://www.youtube.com/watch?v=6kmKbkPbG9E \^o^/

I don't get "하늘을 내려라." I guess the sentence comes from a poem or an ancient literature.