Some parts of some of the designs are transparent. It can be a transparent layer that is placed over a photo, or an illustration/image that is slightly made transparent.
To see if a layer has transparency, you can click on that particular layer and look at the percentage of transparency. If it say 100% it is not transparent. Anything less than 100% has transparency.
If you want to make a particular layer more or less transparent, just change the percentage and see if you like it. It is that simple!