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It describes the scene, tells you how many faces are visible, whether or not the image is focused, lighting conditions.

https://www.youtube.com/watch?v=8CAafjodkyE

They can also generate alt text for photos which do not have that information already. Here's a video of the person in the original article describing this feature:

https://www.youtube.com/watch?v=FLBJUHkLTTA