Although 4K video has been available on YouTube for some time, Google has waited on incorporating HDR. Only now are TV makers baking it in as standard, while popular gadgets like the Chromecast and PlayStation 4 recently got the feature.
High Dynamic Range, in simple terms, doesn't change the number of pixels that you see, it just gets more out of them. HDR offers a wider range of contrast and brightness than standard HD or UHD, allowing the resulting images to show more detail in darker parts of the screen and highlight a wider range of colors. You should be able to pick out details that you may not have noticed before.
Google worked with a number of YouTubers -- including MysteryGuitarMan, Jacob and Katie Schwarz and Abandon Visuals -- to have premium HDR content ready for launch, but anyone can now upload their visually enhanced video.
Starting today, you can watch YouTube videos in HDR on supported devices, such as HDR TVs with the new Chromecast Ultra, PCs hooked up to an HDR monitor and soon on all 2016 Samsung SUHD and UHD TVs.