It is hard to describe without using other advanced words. Sullen = gloomy = downcast = bitter when used to describe a person's mood. It means that person is not very happy and it's pretty obvious, but they're not talking about it or acting on it. They are just quietly being in a bad mood. That is the strict definition that comes to mind. In practice, I would be more likely to use if I thought the person didn't have a good reason to be in a bad mood. The word suggests a little bit that the person is being childish.