Imagine playing a game of catch with your friends. Now imagine instead of a ball, you are throwing a toy airplane. The toy airplane is a drone and it is controlled by someone sitting far away in a secret room. This person can see everything the drone sees and can use it to watch things from very far away.
Now sometimes, these people sitting in the secret room use the drone to attack bad guys who are doing bad things like hurting others. But sometimes, they make mistakes and attack people who are not bad guys. They call these people "civilians."
Civilians are everyday people who are not soldiers or bad guys. They could be moms, dads, kids, and grandparents who are just living their lives. But when they get caught in the drone attack, they can get hurt or even die. This is called a "civilian casualty." It is a very sad thing that should not happen, but it sometimes does.