PWM = Pulse Width Modulation, a way of changing the power of a signal to ‘emulate’ a different voltage or amperage by rapidly varying the ‘width’ of the signal, at a high enough frequency that you shouldn’t notice it.

Basically, it ‘stutters’ the signal. That’s all well and good until it starts to get into the frequency that users are sensitive to (Which is different for everyone). At that point it can cause headaches, nausea, eye strain, migraines and possibly even increasing any epileptic effects.

The other issue is normally it’s fine, but maybe when the brightness is down really low, you might get a 120Hz ‘flicker’ sort of like an old Flourescent light fixture, as it turns off and on the LED backlighting slow enough that you start to actually experience it. For me, I’ve not noticed it even at very low brightnesses and dark rooms, but I’m not sensitive to it at all.