We're talking about the update to Android 6.0 Marshmallow, which is now in the process of rolling out to a Shield Tablet K1 unit near you. It's a 665MB download, and as you may expect it features everything that Google's baked into the 6.0 release of Android (including Now On Tap).
Alongside those goodies, however, you're also getting a new Nvidia Shield Camera app, complete with Material Design, improved burst functionality, and new real-time HD image effects. And thanks to Adoptable Storage, you can integrate the space on your microSD card with the main internal storage of the tablet. Finally, some new wallpapers should be in too.