Deep down in your brain, there's an area called the substantia nigra. Some of its cells make dopamine, a chemical that carries messages around your brain. When you need to scratch an itch or kick a ball, dopamine quickly carries a message to the nerve cell that controls that movement.
When you have Parkinson's, the cells of your substantia nigra start to die. There's no replacing them, so your dopamine levels drop and you can't fire off as many messages to control your body.