Inside the category you can create multiple albums with photos. The albums can be sorted manually using the drag and drop feature or by clicking the move button (blue arrow). In addition you can also copy and move albums between categories.
Each album can also be published or unpublished, just like many other things in Contao.
In the album settings you have to enter at least its name and the source files. All other fields are completely optional.
The date created field can be used to store the information about when the album has been created. It also has another function as this value can be used for the sorting of the albums in the frontend.
Right below it you can define the teaser text and the teaser image which will be displayed on the album list in the frontend.
Beside the teaser text you can also add the full description for each album. It will be presented on the album reader page.