Lately, YouTube has been pushing a ton of server-side changes to users. In just the span of a few weeks, the comments section has been tweaked, the bottom nav bar test returned, and expanded replies were improved upon. The video-sharing service isn't stopping there; today, it seems that YouTube has begun displaying to a few users little snapshots of the video you're watching when you seek through it.
left: YouTube on the web. right: YouTube app on Android.
Frequent users of YouTube on the web and iOS will instantly recognize this feature - it's been out for a while on those mediums, and it's extremely helpful if you're trying to skip to a certain point in a video. I was pleasantly surprised when I saw these previews pop up on my V20 today. After all, apps exist to make things more convenient, and this definitely makes it easier to find a specific scene or frame.
I checked a bunch of my phones to see if they all have the feature, but only my V20 currently does. All are on YouTube version 12.11.57, indicating that this is a server-side test.
Oh, and those screenshots you've just seen? Those are from the Moto G5 Plus review on the Android Police YouTube channel. Check it out.