A full body scanner is a device that you stand in that takes a picture of your body. It works by sending out harmless radiation such as x-rays or radio waves and then makes a picture from the energy that bounces off your body. This picture is then used to make sure that nobody is hiding anything that is not allowed, like weapons or other dangerous items. It is like a see-through picture of what you look like so it can see whatever you have inside your clothes.