Imagine you have some toys that you like to play with. Now let's say that some other kids in your school also want those same toys, but they might not play nice with them. They might use those toys to hurt other kids, or to take things that don't belong to them. That's kind of what an arms trade is like. It means that grown-ups are selling weapons, like guns and bombs, to other countries.
But some people think that this is a bad thing. They believe that selling weapons can cause a lot of harm and violence. That's why they have started a "campaign" - which is like an organized effort - against the arms trade.
The people in this campaign are trying to make others understand why selling weapons to other countries is not a good idea. They want to show that war and violence can have really bad consequences, and that it's important to try and find other ways to solve problems.
They're trying to convince the grown-ups who make the rules to stop selling weapons, and to make the world a safer place. It's kind of like when you ask your parents to take away a toy that you know is dangerous, even if you really like playing with it.