Okay kiddo, so limbers and caissons are like special carts that are used to carry important things. Think of them like backpacks, but for horses or mules to carry around.
The limber is like the backpack for the big gun called a cannon. It helps the horse or mule pull the gun across bumpy terrain. The limber has wheels underneath it so it can move smoothly and the gun sits on top of it.
Now, a caisson is like a special cart that carries important things like ammunition or supplies for soldiers. It looks kind of like a limber but it doesn't carry a gun. Instead, it has compartments inside that hold all the important stuff. It also has wheels underneath it so it can move smoothly.
Both limbers and caissons are important because they help soldiers move their big guns and important supplies easily on the battlefield. Without them, it would be very difficult to move everything around.