Answer:
Step-by-step explanation:
It's an allusion. It refers to a prophesy that Achilles was supposed to die at a very young age. His mother Thetis, hearing this, was horrified. She took him to the river Styx which was supposed to give eternal life. The problem was, she held him by his heel. Later on, that part of his anatomy, which was not protected was how he was killed by Paris, who likely used a poisoned arrow.
The expression refers, when used, to mean that someone is brought down by the only weakness they have.