For those times your smartphone ends up in foreign hands, having the ability to keep certain content, such as photos and videos, out of their sight could be considered essential. Being Linux-based, Android already offers a primitive, and totally working solution for making certain files and folders accessible only to a file manager app, and nothing else.
It's not only simple, but it will also spare you from searching for a file hiding app. There's not a whole lot you need to, so follow this little guide and be amazed!
The first trick is making a dedicated hidden folder. Just create a new folder and put a dot in front of its name. It doesn't matter how you name it, just put a dot before it. This basically tells Android to forget this folder and never look inside of it. Only file managers and specific system apps are aware of its existence. This means files hidden inside won't show up in the gallery, multimedia players, e-mail clients, office editors and the like.
The second option is to hide the media files within an already existing folder by creating a .nomedia file inside it. This file doesn't have an extension or any content. It's simply an empty file called '.nomedia' without the quotations. This will hide all media files (pictures and video) from any app that tries to interact with them.
Important! Make sure to enable viewing of hiding files and folders inside your File Manager, because you won't be able to reach your files otherwise. Actually, the hardest part of the whole hiding ordeal is finding a File Manager you enjoy using. If your phone or tablet doesn't come with one, try ES File Explorer. It's free, fully-featured, supports 30 languages, and it rates really high on Google Play. It has a bit of a learning curve, but it's a polished app worthy of attention.