It means that a narrow depth of field achieves a smooth, out-of-focus blur in the background of an in-focus, sharp subject ...

Blur the background of an image with Google Photos ... which elements you want to focus and which ones you want to be out of focus. This allows you to blur the unwanted background or foreground.

Also called as Acrylic Blur Effect on Sign-in Screen, the feature delivers a beautiful experience by adding more focus to ... will roll out an option to disable Blurred Background on Login Screen ...

It means that a narrow depth of field achieves a smooth, out-of-focus blur in the background of an in-focus, sharp subject photo. That can be whatever you want it to be.