Pixels are the atoms of the design world. On screen, that's all we've got. One lighted dot at a time creates the text, images, and video we see on every computer, TV, or mobile device screen. Each dot can change (via a combination of smaller red, green and blue dots -- shoosh, I know that blows my theory) into a vivid array of colors. Given this array is limited (vivid, but limited) this also means the possible combinations of images that can ever be displayed on a monitor are finite.
If I have an LCD screen that is, say, only two pixels wide and two pixels tall that can each only display two colors, then the maximum possible number of combinations of those pixels and their colors is 2 x 2 x 2 which equals 8. Only 8 possible images can be displayed on such a screen.
Now, take a 1080p HD display. That comes out to 1,920 pixels wide by 1,080 pixels tall and each pixel can display, get this, 16,777,216 colors. The equation of total possible combinations or image possibilities therein is 1,920 x 1,080 x 16,777,216 which equals 3.479x1013 or over 34 trillion. That's a lot but a limited lot. If you were shown one image every second it would take more than 1.1 million years to get through all the possible combinations.
My favorite is that the same logic applies to digital cameras. They have chips with a certain number of sensors that each capture a pixels worth of an image each. Each is sensitive to a limited amount of colors just the same as a monitors color limitation. This, in turn, means they have an upper limit of the number of possible pictures they can capture. Seeing that most digital cameras are far higher resolution this number is positively astronomical but still finite.
We use and abuse computers every day and see so much of what pops up in front of us on them as "real" when, in reality, it's just a better and better simulation of reality.