Screens do not generate an entire image at once. Rather, each row of pixels is rendered one at a time until the entire image is rendered. Imagine the screen as filling in a color by numbers from left to right one line at a time.
This happens a number of times per second based on your framerate. A 59.99 hertz screen does this nearly 60 times per second. If your screen has 4000 pixels, it does one pixel every .015 seconds.
Similarly, most digital cameras use what’s known as a rolling shutter. They don’t capture the whole image at once, but instead one line at a time. This is similar to how a photocopier works.
When the rolling shutter is out of time with the framerate of a screen, it catches different cycles of rendering in the same still.
Imagine if you could instantly swap out a series of photos while a photocopier tried to scan them. You’d end up with a similar effect.