What does the sea dragon's tooth represent in the novel, How to Train Your Dragon?

The sea dragon's tooth symbolizes how close Hiccup came to dying, first in the mouth of the sea dragon, then by the sea dragon's effort to set them on fire, and finally by the tooth that flew directly toward Hiccup when the sea dragon exploded. That tooth would have killed Hiccup if his father had not managed to get a shield in front of him, just in time.