"Isn't it bright?"
In Japanese, they often use negative sentences to ask a question, just like in English, except more I think. And if I'm not mistaken, 明るくない comes from the adverb 明るく (adverbial form of 明るい) plus ない (basically meaning not or not existing). So together it's not bright. At least that's how I think of it. Feel free to correct me if I have anything wrong.