The three elements of the painting are the solid color background, a middleground composed of Hans Hoffman-esque rectangular “figures”, and a foreground of the lines and gestures in the painting.
If you aren't feeling creative, the program will make a canvas for you and paint some nice modern lines and shapes. Just click “I don't like decisions”. It's okay, I don't either. Art is hard. Otherwise, click “Rectangles!” a few times, and maybe add some lines. It's up to you.