What does this icon showing corners of a rectangle with a star on some of my photos mean?
This icon means iPhone took an additional photo from the wider angle camera than the one you selected to give you the option to include more photograph beyond the edges of the original.
To include parts of this extra photograph beyond what’s visible, choose Edit, select Crop and drag the crop handles out to include this extra data in the picture.
If you don’t edit the picture within 30 days, the extra data is discarded to save space. You can disable this option in Settings → Camera → Photos Capture Outside the Frame.
For more information, see the Apple support article on this topic: https://support.apple.com/en-gb/guide/iphone/iph8692d3298/ios