Albums and folders can be hidden from the app so they do not display or played during slideshows.
To hide any album or folder, go into the album you want to hide and select the … menu in the upper right corner. Then select “Hide Album/Folder” from the menu.
If you ever want to see the album again, select the … menu on the app’s main screen and select to “Show Hidden Albums”. This will temporarily display the hidden albums so they can be toggled back to shown.