Squarespace's "IMAGE CAPTIONS" play a dual role of adding visible text under/over the image— which would be a typical caption, and the text is also used to provide ALT-TEXT to the image.

Because you wouldn't want every image to have a visible caption, BUT YOU WOULD WANT TO SUPPLY EVERY IMAGE WITH AN ALT-TAG, you can just turn off the visibility of the caption, which keeps the alt-text intact.