When you first create a gallery the images you upload go to a Folder within your gallery. Now say you have just shot a wedding and have photos from the dressing room, the first look, the ceremony, the after party, etc.. You can upload all those events to different Folders! You can name each of the Folders based on what the event was: "Dressing Room", "First Look".
In Preview Mode, this is where the Folders will appear:
*Remember that you can rename these Folders at anytime.
You can click on any one of your Folders and Upload images, delete images, etc.
Another feature that you have the ability to do is drag and drop Folders to the order that you want.