YouTube on Android is testing video frame previews for users when they adjust the position of the video. Similar to how you can see a preview of the frames in your video when you mouse over a particular spot on the timer bar, the Android app will now show these snapshots of the video's different frames when you use the seek feature to move to a new time in the video, which will make it easier for users to see where exactly they are when fast forwarding or rewinding to a different spot.
This appears to be a new server side change that is rolling out to users in small batches which means that the majority of users on the YouTube app likely don't have access to it yet, but since it has been available on the web platform for a while users should already know what the feature looks like. The seek feature in itself is an added benefit of the app that is a more precise way for getting to a specific part of a video, but it's arguably easier to find just the right section when you're able to view different scenes as you search to give you a general idea of where you're at, though this might be more useful if you're already familiar with the video in some way or know a specific scene that you're looking for but don't know the time at which it appears.
Users being able to see this feature in the YouTube Android app seems to be hit or miss, so for example if you have multiple devices it could show up on one but may not show up on another. If Google ends up keeping this feature around and rolls it out to a wider range of users on the Android platform, it's unclear when exactly that may be. That being said if you use the YouTube app often enough you might notice it show up in the app at some point. There is also the possibility even if it is a small one that Google may roll this feature back as they have done so with some features in the past, but being that it already exists on other platforms it might not be likely.