Imagine a house, but this house is not normal, it's a big machine monster that needs to eat wood or coal to stay warm and cozy. The mouth of this monster is called a firebox. It looks like a small room inside the machine, where you put the wood or coal to make a fire.
When you light a fire in the firebox, it creates heat and energy that the machine uses to do things like making hot water or heating the house. Just like when you eat food, your body turns it into energy that makes you move around and play.
The firebox is made of special materials that can withstand the heat of the fire, so it doesn't burn down the machine. It also has little holes called vents that help the fire keep burning by letting air in, just like when you blow on hot coals to make them brighter.
In summary, a firebox is a little room inside a big machine monster that eats wood or coal to create heat and energy that helps the machine work properly.