YouTube rolled out HDR support back in 2016, but it was only limited to pre-recorded content. Now the company has expanded HDR support to livestreams as well, enabling "vibrant experiences in real-time".
HDR, or High Dynamic Range, offers a wider range for your content and lets screens that support it display darker shadows and brighter highlights. If you are a creator and want to stream live in HDR, you'll need to use the encoders listed here.
Viewers, on the other hand, will need compatible devices to watch live streams in HDR, like the YouTube app on HDR TVs, Android devices with an HDR display, Chromecast Ultra, and Windows and Mac PCs with an HDR display and HDR graphics support.
If your device doesn't support HDR, you won't see the HDR option in the video quality settings menu, but you'll still be able to watch the video in SDR.