In Short: It seems like Google is testing a new Google Search mobile scroll overlay, at least according to a post on Twitter. Now, according to the user who stumbled upon this feature, the tab to scroll will only pop up if you drag your finger up and down the screen. If you press on the scroll tab, the app will bring up the subtopics, which you'll be able to scroll through. The user who found this out says that the feature is not yet all that stable, there are some jittering involved while scrolling. The user who stumbled upon this feature is actually a beta tester for the Google app, and he is using the Google Pixel 2 XL running Android 9 Pie, so this feature could be limited to only some Google app testers for now, as it doesn't seem to be available for the general public just yet.
Background: Now, Google recently announced some new Search features that will change the way we look and find content on the web, according to the company. Google is looking to focus more on personalized results moving forward, and the company has added features like 'activity cards', which will show you websites that you've already visited, and are related to what you're searching for at the moment. 'Dynamic organization' is also a feature that Google introduced recently, and it will basically organize your search results which provide most relevant subtopics which are specific to what you're searching or at the moment. This new feature, that was spotted in the Google app, seems to be a part of what Google already announced, a new feature that will build upon those changes.
Impact: Google is bringing some serious changes to the Google app, and the way we search for stuff. Personalization is usually a good thing, as search results will be more tailored to you, and hopefully more accurate because of it, though only time will tell, of course. This new feature is still not ready to roll out to everyone, and it's even possible that not all beta testers are seeing it, so it may take some time before it makes to the stable Google app.