This function allows you to hide any photos that are getting in the way but that you don't want to delete. An example could be a photo of a receipt or document. Hidden photos are hidden from all views.
You can watch the following the following video to see how to hide pictures:
To open up your hidden photos, go to the timeline and open "View & Filter" (at the top right on mycloud.ch and at the top right below the three dots in the mobile app). Then turn on the "Hidden" filter.