its when youre exposure (the setting used to control the amount of light taken into the camera) is adjusted automatically on the fly, to brighten up the shot in darker situations, and darken them in light ones. for example at 0:35
just go into your menu and turn on manual exposure and adjust it. it will make your videos more professional looking so to speak and try not to chop heads/feet so often. some of the clips were right on him and some were 10 feet away