What does exclamation point on photo mean?

Hi. I believe it means that it's a compressed (lower resolution) copy of the original, which is stored in iCloud. It is done to save storage space on your device, enabled by the Optimize option in Settings > Photos. If you want to use that photo, it will need to be restored from iCloud, which may cause a delay.

Thanks for the explanation. What does "need to be restored" mean if you are trying to get you photos onto your Mac? How does one do this? Link?

Hi. Try accessing your iCloud Photo Library directly using the computer.