Well, the brain has billions of tiny little cells called neurons that take messages around the body. When you think about moving your hand, it starts in your brain where you make the decision, and then some neurons travel through your shoulder and arm to your hand to tell the muscles what to do. Instead of using words they use a kind of electricity, so if machines use the same kind of electricity they can trick the brain into thinking they're neurons too. So if the paths that normal neurons use to travel are broken and they can't get across, we can program the machines to get through instead.