system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does streaming videos with a resolution larger than my screen work?

For example, streaming a 4K video on a 1080p monitor, or a 1080p video on a 720p laptop screen. I've never been able to wrap my head around this concept.
gpt
Imagine you have a 40x40 pixel black/white image, and you want to display it on a monitor that can only display 20x20 pixels. Let's say every second pixel is black, so you get a chequered pattern. 

Leaving out every second pixel is not an option - it would work well in this specific case because of the halved resolution, but it would also result in a wrong picture (completely black, because you would skip every white pixel)... and cases with not-so-nice differences in resolution would get complicated. 

A better method would be taking 2x2 pixels, calculating an average color value for that area (grey, in this case), and displaying it as a 1x1 area in your new picture. This will *basically* do the job (chequered original picture would look grey, if you aren't looking at it from up close), but won't look very nice in the end since small details will most likely just get "blurred out" - but it's the basic concept... you will take certain areas, and recalculate their color values based on what they should look like "shrunken down" to the new resolution. There are several different algorithms that do exactly that (bicubic interpolation, Lanczos algorithm, ...) while trying to conserve as much image quality as possible - of course they are more complex than simply averaging out color values.