What does the key photo option do/for?

I am fairly new to iPhone I am very tech savvy. I know everything about androids, but this is my first iPhone and a feature just popped up where I could select the key photo if I selected I am not sure what that does or what it’s for I just searched on Google, but I could not find a basic simple answer. I am using the iPhone 14 with the most current iOS. I was just in the Camera app and I selected something where the box popped up on the bottom right corner of the screen that said key photo but I did not select it I wanted to search and find out what it is for first. Also I have an apple watch series 8, and for the last couple days when I receive a text message and no longer shows on my watch. Normally it vibrates makes a noise I look at my watch and I can see the text message but it no longer does that it was showing a red dot at the top that once I hold down the screen I would see it but now it’s not even doing that, I asked a friend of mine if hers is working and she said no it’s doing the same thing so my first thought naturally is I’m sure Apple will fix the issue. It might’ve been something with the last update?