You indicate that your using macOS Ventura, so I'm guessing you mean the app Photos rather then the old discontinued programed called iPhoto. Is that right?
Use command-I to open the information window. In there you will find a text field for entering a caption/description. You can type what you like, but use option-return to to add a new line.
The Info window is full of good stuff. It shows the file name, but you can add a Title that would be more descriptive. The title will appear under the thumbnail of the picture. I use a title like "22 08 17 Aunt Ellen at Grand Canyon" or something like that, with the date leading to make it easy to sort. If there are lots of pictures of Aunt Ellen at the Grand Canyon, I might use "22 08 -01- Aunt Ellen at Grand Canyon," using an index number to put them in order.
You can also add keywords like "AuntEllen" and "GrandCanyon" that you can use in searches. (Spaces are allowed, but I sometimes find them confusing.) Keywords are among the most powerful things you can use to help organize your pictures.
The Info window will also show tiny icons of faces it identifies in the pictures, if there are any. I use keywords to list people I know in the picture, but you can add names of those who Photos recognizes as people. It will say "unnamed" under a person's face, but you can type in the correct name, and then the people will be listed in the People view. You an also add names for people that Photos didn't recognize.
Check this out at:
Find and identify people in Photos on iPhone - Apple Support
