As the title implies, it depicts three spheres resting on a flat surface.

The sphere on the left is transparent (see ball lens) with a photorealistic depiction of the refracted light cast through it towards the viewer and onto the flat surface.

Its reflection is a self-replicating image of Escher in his studio drawing the three spheres.

In the reflection one can clearly see the image of the three spheres on the paper Escher is drawing on: in the center sphere of that image, one can vaguely make out the reflection of Escher's studio, which is depicted in the main image.

The sphere on the right is opaque and diffuse, i.e. neither specularly reflective nor transparent.