Take you cell phone and consider one pixel. You could take that pixel and scan through every possible color value. Now if you did that for every combination of pixels, what you'd be left with is every possible picture you cell phone could display. But of course your cell phone has a camera on it, which means you could take a picture of things, and they appear on your cell phone screen.
The point is by systematically generating every possible image your cell phone can display, you have generated every possible image.
Once we're finished with that we can begin on generating every sentence ever written.
Do Androids Dream of Electric Sheep, East of Eden