In this project we are going to turn an LED on a breadboard on and off by using a pushbutton switch on the breadboard. The switch does not directly control the LED; instead, the switch sends a signal to the Arduino which then turns the power on or off directly to the LED.
It may seem unnecessarily complicated to use a switch to tell a computer to turn a light on or off. Why not just directly connect the switch to the light?
While that may be true for this exercise, in many real-life situations the manually actuated switch can be replaced by some other automated signal. The ability for the Arduino to intercept that signal and then initiate some other action is an important component of many types of industrial or commercial automation projects.
Now that you've wired up a few projects, try to build this one just by looking at the diagram above.