Hidden Songs/files means you will hide that song or file from your Playlist, but the hidden song/file is not deleted from your Device.
If you want to hide songs, you can go to the “Settings” page, and tap “Hidden Songs” in the center of the “Settings” page.
Unhidden songs/files means that you will show the song or file in your playlist again.
If you want to Unhidden songs/files, you can go to the “Setting” page, select “Hidden songs”, then select the songs or files you want to unhide. In addition, you also can go to the bottom of the music list page, then “See Hidden Music” will be displayed.