Who is Achilles and what was the importance of his heel?

Achilles Heel is an old saying about a guy who is invincible except for his heel. the moral is that everybody has a weakness. Example: The spiders are her Achilles Heel or it means: the only thing that can scare her are spiders .

His mother dipped all of Achilles in the river Styx all except his heel. He was invincible everywhere except his heels because of that