We were first introduced to Photos with iOS 8, which included features such as hiding photos as part of the stock photo manager on iPhones and iPads. In Photos for Mac, however, "Hidden" photos don't work like many people expect them to. There are many reasons why photos may need to be hidden, so it's a good idea to take a moment and understand Apple's idea of "hidden" when it comes to Photos.
In short, "hidden" doesn't mean what you think it means. You won't see a hidden photo in Moments, Years, or Collections. If you have that photo in any Albums though, it's still sitting there just like all the others.
Hey look, it's Lego minifigs having adventures!
If you hide a photo on iOS, you get a lovely dialog that tells you exactly what I said above. In light gray text on a white background. You know, <sarcasm>so it's totally clear what's happening</sarcasm>. And if you're on a "short" iPhone? Forget it. I had laser eye surgery and with 20/15 vision in both eyes, I had a hard time reading that dialog on my iPhone 4s. It's only marginally better on a size 5 iPhone, so basically unless you have a 6 or 6 Plus, you might not even know it said anything above the "Hide Photo" and "Cancel" options.
Here's what happens on your Mac: You are in the main "Photos" view and see a photo you want to hide. Right-click and choose Hide Photo. You get a dialog asking to confirm you want to hide the photo (but only kind of):
Why you'd hide a photo of Reggie Lee with your Lego guys is beyond me. But it's just an example.
It's worth noting that in Photos when you hide a photo you simultaneously create a new "Hidden" album where all those pictures go. That album is right there with the other albums in the app. Because again, what better way to hide photos than to draw attention to the fact that they have been hidden?
The Hidden album. HEY LOOK! I'M HIDING!
On the upside, if you only ever use this "Moments" view, you're golden, the photo isn't shown:
Hey look, that photo I wanted hidden is gone!
But then if you go look at an Album that has all your pictures in it, there's the "hidden" photo, actually not hidden at all:
Hidden photo is not hidden. What, you don't take your Lego guys with you to live re-enactments of Star Trek episodes?
How this is hidden, I can't say since it's only hidden half the time. Or in some cases, if you're someone like me who only uses Years, Moments, and Collections because you select them accidentally, not hidden at all. So if you're wondering why clicking on "Hide this photo" isn't getting you anywhere, at least you know it's expected behavior. Apple's definition of "hidden" however, I don't have any way to explain that.