Imagine a big playground with lots of kids playing different games. Now imagine that sometimes, some of these kids get rowdy and start shouting or making loud sounds that disturb the others. To prevent this from happening, the teachers in the playground decide to make a rule that says: "no shouting or screaming allowed."
In the same way, the Comprehensive Test Ban Treaty (CTBT) is like a big rule that grown-ups made to prevent countries from making loud and dangerous sounds that can harm others. But instead of shouting or screaming, they made a rule that says "no nuclear explosions allowed."
Nuclear explosions are very dangerous and can cause a lot of damage to people, animals, and the environment. That's why a long time ago, many countries decided to work together to create a big treaty that would ban all nuclear explosions everywhere in the world.
The Comprehensive Test Ban Treaty is the name of this treaty, and it's like a promise that all the countries who signed it will never test nuclear weapons again. This doesn't mean that they can't have any nuclear weapons, they just can't test them to see how powerful they are.
To make sure that nobody breaks this promise, there are people called "inspectors" who go to different countries and make sure they aren't testing any nuclear weapons. These inspectors use special equipment to detect any sounds or movements that might indicate that there's been a nuclear explosion.
By having a comprehensive test ban treaty, the world becomes a safer place for everyone, just like how the playground becomes a happier place when everyone follows the rules and plays nicely.