a red 1 on Apple Photo icon

There is a red 1 on the Apple Photos icon in my dock. I can't seem to figure out what it refers to. I have read other support posts about this but none of them fully answer my question. I understand that it must relate to something new which I have not seen. Perhaps it is a notice of some sort, or an indicator that I have a new photo, shared by a friend? But I can't seem to determine what it refers to. Can anyone tell me how to find out what, exactly, the 1 is referring to?