It only affects a small, unfortunate portion of the userbase, at least as far as we know. If I had to poorly explain, it’s because OLED screens primarily only use the red and green lights to control brightness, so it flashes the blue light at a high, unnoticeable frequency to give off the impression that the blue lights are of the same brightness as the red and green ones. Normally this isn’t a problem, but in the case of the iPhone X line, it’s abnormally low—something like 220Hz, which on most devices is far into the thousands.