The Rule of simplification means having a more detailed/ lighter center in the image, while the background is less detailed/ darker. This draws more attention to the center of the image, or the subject that you would like the viewer to focus on. To do this, you will need to use a large aperture on your camera.
The image above is an example of the rule of simplification. The flower in the image is more detailed and lighter, where as the background is out of focus and darker. This draws the attention of the viewer straight to the detail of the flower in the image.
However, the image above shows the opposite to the rule of simplification. By allowing all of the image to be in focus, it gives the impression of business and chaos, which is relevant in this image as it conveys the business and chaos of a city life.