This is a section of a retina, which has thickness of about ¼ mm in most places. There are piled up with many organs, which we call cells. At the bottom there are receptors, and then bipolar cells, and ganglion cells.
When light comes from outside it goes through the retina and reaches the outer-segments of receptors where the light is absorbed and produce electric response. The light is changed to electric response and the response is transmitted to the bipolar cell and to the ganglion cell, and finally to the brain.
There are two types of receptors, rod and cone which are responsible for dark environment like night and bright environment like daytime.
A problem here is that the incoming light has to pass many cells before it reaches the final goal, receptors. The is inevitably absorbed, scattered, and refracted on the way. The final retinal image is deteriorated. It is much better if the light can reach directly receptors without passing other cells. How can this be done?
The answer is this. All other cells are pushed aside into all the directions as you see here. But it is possible at one position on the retina and there happened to exist a hollow. So the hollow indicates the center priority of the retina.
You may say that how about to invert the retina up-side down so that the receptors comes upside and light can directly reach the receptors? Like this.