What does it mean when they talk about gnashing of teeth?

In The Bible when they refer to the gnashing of teeth, it means that they were in great anguish. They would tighten up their jaws and groan or cry. Picture when a pirate in a movie gets run through with a sWord and they grimmace and say "AAARRRRGGGGHHHH!" with their mouth clinched shut in pain. I know it's silly but it's the first picture that comes to my head when I think of the gnashing of teeth.