system
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human
ELI5 : How does oversampling work?

I just don't got it. I'm talking about oversampling on a screen. Let's suppose I have a full HD screen. That means 1080*1920 pixels. When a game purposes to display the image with oversampling, how does it work? My screen still has the same amount of pixels doesn't it? How can it displays a more detailed picture?
gpt
What is the benefit of oversampling? You get a cleaner image for edges, color gradients and transparency. 

If you calculate the imagine in a higher resolution first, that imagine contains more *information* about what is going on on the screen in comparison to the lower resolution. 

So if you get a FullHD image you get just 1920*1080 pixels, let us assume you calculate a scene in a dark hut in shadows and a fire that throws sparks. Now take some pixel that there is SHADOWY_BACKGROUND and the next pixel to it is RED_FLAME. So you get a sharp transition, a jump from BLACK to RED between the shadow pixels to the left - they are all very dark grey - and the next one, that is a bright red spark. It clearly is discernable as a black surface next to a red block, especially considering the again next pixel is also the same (or very similar) red. 

Now you do a oversampling in 4K (this is a good example, as you get exactly twice as many pixels in each dimension): The same transition from the dark shadow to the flame might not be as clear cut. You have a dark shadow, the next pixel is a shade of red, the  next one brighter, the comes the red pixel.

You have the same scene, but with more information, more details about all color gradients, all transparencies, and all edges. If the oversampled image is downsampled, it is not just done by discarding, for example, every second pixel, but the image is processed in way that pixels next to each other are averaged. So instead of BLACK BLACK RED you'd now get BLACK DEEP_SHADE_RED RED in the FullHD image, which means your oversampled FullHD image is cleaner regarding *all* color gradients that are next to each other than a native FullHD would have been. 

In this way, oversampling works like a full-screen all-details anti-aliasing. Just with the difference that it works with internally more details that the native resolution would provide. Normal anti-aliasing basically works by blurring the image in smart ways, oversampling works with more details and then uses that to average near pixels instead of just "blurring around".