Negative painting is simply painting the space that is not the subject. If you put your hand on your hip the space created between your arm and your body is negative space. The shapes between the rungs of a chair are negative space. The shapes between the branches of a tree are negative space.
This is a technique to create depth in your artwork. As shapes of any kind overlap, the one in front moves forward and the overlapped shapes move back.
While the painting I used as an example is abstracted, you can identify which trees are close and which are far. Because it is abstract, I disregarded the concept of cool colors receding and warm colors advancing. I will address this concept in a separate post.