To add a description to your album or edit the existing description, from the Organizer, open the album dropdown and select Edit Album Description.
If you want to edit the description without opening the album, you can use the album dropdown in the side menu and select Edit Album Description.
A window will open where you can add or edit your album description. There are many editing tools where you can customize the color, size or font of the text.
Now, when you view your album in the Gallery, the description will appear above your photos. This is a great way to give your visitors some context to the album they are viewing.