Defining the surface of a molecule turns out to be a nontrivial task with no unique solution. Apart from the implications of quantum mechanics ("atoms have no boundaries"), different "objects" will "see" different surfaces for a given molecule.
A Solvent Excluded Surface (SES), which we see here, is defined as that part of the molecule that would be "visible" to the hull of a water molecule: the molecule is considered as a union of Van-der-Waals spheres, over which a water molecule is rolled. The SES consists of the points where the surface of the water touches the surface of one Van-der-Waals spheres. The SES we can see here is colored by the element of the nearest atoms.