Administrators are given two unique tools that can only be used by them when logged in with the Administrators email address linked to the Gallery and using their Admin passcode. Hiding photos is useful when there are multiple photos of a similar shot, and allows visitors to focus on your favorite of a sequence. And of course, any other pictures you don’t want to have visible to other gallery visitors. They are still accessible to you, just no-one else. Hovering over a thumbnail, you can see the icon below the photo with the eye and slash.
In addition to hiding from thumbnails, you can also see the hide icon when the photo is fully open. Note, when you hide a photo, the share icon is grayed out and prevents you from sharing by accident. If you want to share, just click the hide icon again to unhide.