There have thus far been three devices release in 2017 with support for HDR video: the Galaxy S8, the Galaxy Tab S3, and the LG G6. There isn't a lot of HDR video out there, though. Netflix is one of the few sources, and now it's finally available on Android. However, only the LG G6 is supported right now.
You need several things in order to watch HDR and DolbyVision video on Android right now. At the top of the list, of course, is a G6. Then, there's the new v5.0 update to Netflix. It's still rolling out, but we've also got it on APK Mirror. Lastly, you have to be on the 4-screen streaming plan, which is $11.99 per month. Most people are on the 2-screen plan for $9.99, but the higher tier is required for 4K and HDR video.
Titles with HDR support will be marked as such in the app—many Netflix originals have HDR streams including Daredevil and House of Cards. There does not appear to be anything else different in this version of the Netflix app, so there's no need to scramble to get this update unless you've got a G6.