An Arduino is an open-source micro-controller development board, which can be used to read sensors and control items eg. lights, sensors, motors and monitors.
Programs are uploaded to the Arduino board which can then interface with things in the real world.
For example, you can program an Arduino to make a light show to music, or activate movement for a robotic arm, or create a plant watering system, or create a device to limit your time in the shower, or create smart bins where you are notified when they are full.
The possibilities of the Arduino are almost limitless.