How to hide cards in your mind
Everything in your mind is already private, but if you're opening your mind in public and don't want someone seeing a specific card over your shoulder, you can hide it from your search results and overview.
Just click on the card and get into the details view. Now press ENTER or click +Add Tags and add the tag Private, NSFW or Hidden. Any cards with these tags will be hidden from your overview and search results.
Now these cards will ONLY appear when you search for that tag (ie. type "private" in the search bar and hit enter). You can always unhide them later by removing the tag.