What does it mean to have an "achilles heel"? how did this saying develop from mythology?

The Achilles heel was first heard of when a child named Achilles was dipped into he river styx in the underworld. He was held into the waters by his heel. His heel was the only thing holding him to humanity and mortality so it was then his only weakness. He couldn't get hurt anywhere but his heel. Achilles died when he was shot in the foot by an arrow. 

According to the lore you can get an "Achilles Heel" by bathing in the river styx, but you must choose your area of weakness.