1. Used alone to tell someone to stop doing something (which usually involves trying to get your attention) and to wait until you are ready to attend to them. English = 'Hold on!', 'Hang on!', 'Wait a sec!' and so on.
1. Used to introduce a statement which contrasts with what you have just said. English = but, yet, although.
2. Used alone to show the person you are talking to that you are thinking and about to say something. English = umm, err.