Android 9 Pie rolled out yesterday, officially. And it apparently brought back a feature to the original Pixel and Pixel XL, that were taken away in one of the recent betas. And that was Quick Text Selection in the Overview Screen. This is actually a pretty big feature in Android 9 Pie in the first place, but it was removed in one of the recent betas for the first generation of Pixels, and was not expected to come back. As Google had noted in its bug tracker that it would not be fixed. But it has reappeared in the final version of Android Pie that rolled out this week.
Quick Text Selection allows you to quickly select text while in the Overview screen. This means that you can go to the Overview Screen and find text in another app that you have open, long-press to select it and then copy it into another app. This can be very useful for copying an address from another app and into Google Maps, if you are looking to find directions to someplace. It's a feature that you may not use all that often, but it does make it much easier to copy and paste text from one app to the other, without having to actually jump back into that app.
Now according to many with the Pixel or Pixel XL, the Quick Text Selection in the Overview screen is only working on devices that have the system language set to US English. For some reason, it is not working with other languages. That is likely just a bug that will get sorted out pretty soon, if not in the Android 9.0.1 update. But as long as you are using your phone in US English, it should work without much of an issue, which is definitely a good thing here. If you haven't received the update to Android Pie yet, you should be receiving it in the next few days.