Arduino makes it as easy as possible to program tiny computers called microcontrollers, which are what make objects interactive.
You are surrounded by dozens of them every day: they are embedded in timers, thermostats, toys, remote controls, microwave ovens, even some toothbrushes. They just do one specific task, and if you hardly notice them - which is often the case - it's because they are doing it well. They have been programmed to sense and control activity using sensors and actuators.