The Desktop folder contains all the files which you see on your desktop. The default path is usually ~/Desktop but you can change here the location of this folder. All the content will be moved to the new location when you click Apply or OK.

The Autostart folder contains applications or links to applications or scripts you want to start whenever KDE starts. You can change the location of this folder if you want to, and the contents will move automatically to the new location as well when you click Apply or OK.

This folder will be used by default to load or save documents from or to.