The term is derived from the Greek myth about the hero Achilles, who was said to be invulnerable except for his heel. His mother, Thetis, had dipped him in the River Styx as an infant, making him invulnerable to harm except for the part of his heel that she held onto. In battle, Achilles was eventually killed when an arrow struck his heel.
In modern usage, "Achilles' heel" is often used figuratively to refer to someone's one weak point or vulnerability that could lead to their downfall. For example, someone who is very talented at their job but has a poor work ethic might be considered to have an "Achilles' heel" in their lack of motivation.
The term can also be used more broadly to refer to any potential vulnerability that could jeopardize a person's success or well-being. For example, a country that is heavily dependent on a single resource for its economy might be considered to have an "Achilles' heel" in its vulnerability to fluctuations in the global market for that resource.
In general, "Achilles' heel" is used to describe a weakness that is difficult or impossible to overcome. It is a reminder that even the strongest and most capable people have vulnerabilities that can be exploited.