Okay kiddo, have you ever heard of rules that grown-ups follow so that everyone is safe and fair? Well, an arms embargo is a rule between countries that says they can't sell weapons or military equipment to each other.
You know how sometimes people might have fights and arguments? Well, arms embargoes are put in place to try and stop those fights from getting worse. You don't give somebody a stick to hit you with, right? That's sort of what this is like.
Sometimes countries might not like each other or disagree about things, and if they were allowed to buy weapons from each other, it could make their problems even bigger. So, the countries will all agree to not sell weapons to each other.
It's like when parents tell siblings not to hit each other. It's there to keep everyone safe and calm so that they can work things out without anyone getting hurt.