The most common use of the head wobble is to respond in the affirmative. For example, if you ask someone if the train is going to your destination and they wobble their head in reply, it means "yes". The head wobble is also often used as a sing that what's being said is understood. For example,if you tell someone you'll neet them at a certain place at 5 o'clock and they wobble their head at you, it means that it's fine and they'll be there.