Well, have you ever heard of the expression "dead as a doornail?" When someone is truly dead, it means their body has stopped working forever.
Every living thing has different parts inside their body that help keep them alive, like the heart, brain, and lungs. When these parts stop working, it means the person or living thing has died.
Sometimes, people can be brought back to life if doctors or paramedics do certain things to help their body start working again. But other times, when someone is truly dead, nothing can be done to bring them back.
It's like a computer or a toy that's broken and can't be fixed. It's not going to work anymore, no matter how much you want it to. That's what it means to be truly dead.