The Arduino is a palm sized micro controller. It is a computer-like device that allows you to control real things in your world with a computer. They are every inexpensive costing around $30. You can build robots, 3d printers, control your heating or home lights, connect to the internet, connect to your phone or a camera. They are open source and are designed to hacked and re-confugured.
A great video if you are interested and know nothing about micro controllers.