Polish (also known as Pol) refers to the smoothness of a diamond’s surface after polishing works - whether there are any polish lines, beard girdle or facet that remained, whether the diamond is polished beautifully and brilliantly. Remains of polish are barely visible with the naked eye, more often than not, they are not visible or findable, so the effect of polish would not affect the overall cut and appearance of a diamond significantly.
Symmetry (also known as Sym) refers to the alignment of facets on a diamond. If the facets of a diamond seriously misalign, for example, if the culet (the smallest and lowest point of a diamond) locates far away from the center of the diamond, or if the girdle is overly thick, etc., the fire and brilliance of a diamond would be significantly affected.
The image to the left shows a serious misalignment of the facets which significantly affects the diamond's fire and brilliance. The image to the right indicates a proper alignment of facets, which can usually be found in a diamond with good to excellent grades of symmetry.