The button is clever. It's located on the bottom half of the case and is not a soft button that presses something on the phone. This button is exclusive to this case and when pressed, launches the iPhone's Camera app even if the iPhone is locked. A quick press takes a photo and a longer press takes a QuickTake video.
The $129.00 case is available for both iPhone Pro 11 and iPhone 11 and has other features too. It's compatible with Qi-certified chargers and works with USB-PD-compatible chargers to pump power into the battery at a faster rate. Apple says when the case is fully charged, it will provide 50% longer battery life.
This is the first time Apple has added a shortcut of sorts to the camera app. On most Android phones, a double press of the power button launches the camera app. It's handy, and while this button is exclusive to a case, it's a step in the right direction. Here's hoping that Apple figures out how to add this button to non-battery cases.