Android Is Upgrading
As many of you know, after each software or security update the phone has to reboot. Once it does you'll have to sit for a few minutes or longer while "Android is upgrading". This is preparing all of your apps for the new software. It's a slow and painful process.
New to Android 7.1 Nougat Google has made this a behind the scenes (in the background) process. Meaning after an update the phone boots right up, and optimized apps in the background and app tray. Rather than making the phone unusable for a few minutes or more.
It's a small change, but a nice one that many will appreciate.