We know the sky is blue and the earth has one moon. But is there life on other planets? Such a question is debatable, meaning it is subject to disagreement or doubt.
If you've ever seen politicians or anyone else have a debate, then you already have a pretty good idea of what debatable means. Something is debatable if there's reason to doubt it. If you say you're going to play in the NBA someday, but you're very short, your plans are debatable. Many issues, like how to create more jobs, are debatable because there isn't one correct answer to them.