Whenever you have a reference whose type is some parent class (like JPanel,) but you know that actually it's pointing to an instance of some subclass (like CharacterPanel), you can use a cast. The cast in this case is telling the compiler that you know something it does not. In this case is would like like
CharacterPanel myPanel = (CharacterPanel) myTabbedPane.getComponentAt(0);
That "(CharacterPanel)" is the cast. You might want to read these two articles, and then I guarantee you'll understand this stuff perfectly:
http://www.javaranch.com/campfire/StoryCups.jsp
http://www.javaranch.com/campfire/StoryPassBy.jsp