Defensible space is like having a barrier between a fire and a home or building to keep the fire away. Just like a castle has a wall to protect it from invaders, a house can have a defensible space to protect it from a possible wildfire.
This defensible space is created by making sure there are no trees, bushes, or other things that can easily catch fire near the house. In fact, the space is usually at least 30 feet around the house and anything flammable is removed from this "zone".
When firefighters come to protect a home during a fire, they look for this defensible space because it helps keep them and the home safe. It gives them a clear area to work in and a place to protect the home from harm.
So, imagine that a fire is like a monster that wants to eat your house. The defensible space is like a fence that keeps the monster away from the house and makes it easier for firefighters to defeat the monster if it tries to come close.