Pay attention to wear you're pointing your gun.
The muzzle of the gun is the front end of the barrel. This is where the ammunition comes out. The command "What that muzzle!" is used to remind a person holding a gun to pay attention to where the gun is pointed. The gun should not be pointed in the direction of people or at other things that could be damaged if the gun were to go off unexpectedly. It is also important not to point the muzzle in a way that could be seen as threatening.