When the red (R), green (G), and blue (B) circles are made very small and placed close together, they eventually become too small for the human eye to distinguish individually. Instead, the eye combines and averages the amounts of R, G, and B light within a certain area, perceiving them as a single color and brightness.
When these tiny RGB dots become smaller than the resolving power of the human eye, the individual colors can no longer be distinguished. Instead, the surrounding RGB light is spatially averaged by our visual system, making the pattern appear much like the original image.