What is meaning of jaw-dropping?

When you are shocked, amazed, disbelieving, astonished, surprised, etc., your mouth may open -- in other Words, your jaw drops. Sometimes it is a natural, spontaneous response, and other times people do it on purpose to be humorous or to exaggerate their reaction.

Susan Boyle gave a jaw-dropping performance on the TV show "Britain's Got Talent." In fact, the judges' jaws really did drop, as seen on the video of the episode.