If you shoot a gun forward while standing on a very fast train, you’ll see it speed off like usual. But from the ground, the bullet’s velocity would add to the train’s speed, making it seem twice as fast. If you turned around and fired the gun in the opposite direction, you would see the shot move toward the rear of the train. And if its speed perfectly matches the train’s forward motion, they would cancel out, making the bullet look like it simply dropped onto the ground.