The App Inventor coordinate system is based on an X,Y grid where X is the horizontal axis and Y is the vertical axis. The upper left corner of the screen is location X=0 and Y=0, or (0,0). The x coordinates get larger as you move to the right and the y coordinates get larger as you move down.

coordinates.png

The Z coordinate allows you to add depth to your image or sprite. Thus a 1.0 will be flat on the screen, a 2.0 will be on top of a 1.0 and a 3.0 will be on top of a 2.0 and so on.