Means that Godot will fetch the content of the variable
muzzle(which is supposed to be a node) and will call the function
add_child from it.
But, if you never told Godot what is the content of the
muzzle, Godot does not know what to do, and will tell you that you are trying to call
add_child on nothing.
You can assign a node to muzzle in many different ways, depending which node is referring to: