A diamond tool is like a special kind of tool that is really, really strong and can cut through things that other tools can't. Imagine a sharp knife that can cut through almost anything, but even stronger than that!
The reason it's called a diamond tool is because it's made of tiny little bits of diamond that are really hard and strong. Diamonds are special because they are one of the hardest materials on Earth, which means they can cut through things that other materials can't.
So when someone needs to cut something super tough, like really hard rocks or metal, they might use a diamond tool because it's the only thing that can do the job. It's kind of like a superhero tool!