Pillow shading is basically shading without a lightsource pretty much. The stuff on the left in the image is pillow shaded, Stuff on right is a better more corrected shading technique, Many newbies in GFX often Pillow shade, Not that Pillow shading is BAD, But it is only good in certian situations, Like if you was to try to pixelate yourself holding a flashlight to a pillow, Light would be in center, Which makes more sense then if you seen light from the corner.