If I gave you access to every room in a skyscraper, could you write a message simply by having the lights on in some rooms and off in others? A computer isn't all that different.
So let's say that skyscraper has automatic light sensors, only I mess that skyscraper up and put the light sensor from room 2 in room 1. So when the lights are on in room 1, they turn off in room 2. So now the light switch in the room 1 controls 2 rooms of light.
The trick from there is to position the light sensors in other rooms so that you can control a lot of light with a simple control. Then let's dedicate one room to a timer, where the light turns on and off repeatedly every 10 seconds. We will call that the clock room, and we can put sensors there. Now we can cause my message to flash back and forth between 2 states.
Basically a computer is just a very careful positioning of lights we can control and lights that are automatically controlled based on the conditions in the ones we control.
[Source: Reddit]