Often we have a project which requires a number of devices at once. Perhaps more than one servo, a display, perhaps a distance sensor, or a GPS unit. The Arduino Uno has enough pins (about 20) to connect all of these at once.

Then we load a bunch of software into our host Arduino. Each device has a code library which helps us perform various functions with that device. The Arduino Uno (and many other similar microcontrollers) generally has enough memory and other features to run all that software at once as well.