A hidden album is one that the media can not see and all images within a hidden album are not 'searchable' via the image library. It will not appear on your profile for anyone other than yourself. 

The hidden option is most commonly used for albums that contain non-current images. You can hide and unhide an album at any time. 

STEPS TO HIDE AN ALBUM

1. Open the album. you wish to hide.
2. Click the cog on the top right corner of the album to access Album Settings. 

3. A pop up will appear, select "Visibility" from the available tabs.
4. Select "No one, keep this album hidden".
5. Click "Save" 

To check that your album is hidden, look for the small white banner under the album name which will state "HIDDEN".