The currently selected album or group's description is displayed above the thumbnails.
You can modify the description (or add a new one) by clicking on the description or anywhere in the area above the thumbnails and below the toolbar.
In this example, you see the currently selected album "Fun" selected on the left. The album's description "A collection of unique or fun pictures" is displayed above the thumbnails. You could click on the description and edit it directly, or you could change it in the Album Properties dialog.