An intercontinental ballistic missile, or ICBM, is a big rocket that can travel really, really far, like from one side of the world to the other.
It's kind of like throwing a really, really big ball really, really far. But instead of throwing it with your arms, you have a rocket that blasts off from the ground and goes up into space.
The ICBM can go super fast and high up into the sky, and then it can come back down and hit a target on the other side of the world. This makes it a very powerful and dangerous weapon.
Countries build ICBMs to protect themselves from other countries that might want to attack them. But some people worry that if too many countries have ICBMs, they could accidentally start a big war and hurt a lot of people. So it's important for countries to talk to each other and try to make sure everyone is safe.