A brief overview of the features in the galleries:
1. Gallery Description
Once you have opened an album, you see the first photo of it as shown on the left image below.
When you click on the button in the lower right corner (the button with the 2x2 squares), a new view slides up (right image above). It shows the album-description and the thumbnails of the all the images included in this album.
2. Show Image Captions and Image Full View
If an image of the album has a caption, a small info-circle is shown in the upper right corner (see left image below). Click on it and a new view slides up (see right image below): Now you can see a full view version of the particular image on the left, and the image-caption on the right.
Press the info-circle again or just press ESC, and you get to the previews image view.
By the way: you can navigate easily between the gallery-images by hitting the left- and right-key on your keyboard.