High Dynamic Range (HDR) has seemingly been the next big thing in video playback for quite some time, but most people have yet to experience outside of taking photos. It relates not to the resolution of a video, but to the amount of contrast and the color range, and it should make things look much more realistic. HDR content is still fairly limited, however, and so is device compatibility. The ill fated Samsung Galaxy Note7 was the first phone to support it, but few people got a chance to try it out on that, for obvious reasons.
YouTube is showing an HDR playback option for some Galaxy S8 and Note8 users was written by the awesome team at Android Police.