Polish is graded the same way that symmetry is, Excellent, Very Good, Good, Poor or Fair. When a polish has a grade of Poor or Fair it means that the facets may reduce the intensity of the light that is reflected from a diamond, which means less brilliance or sparkle. They could also have polish lines that are blurring the surface of the diamond giving it a very dirty look. Every diamond cutter, even the most skilled, can run into variances in the grain that no amount of polishing will remove. It is simply a defect that cannot be removed. No amount of cleaning will ever take this look away because the lines are reducing the amount of light that enters the diamond.
It is found that most people want a diamond that has an Excellent or Very Good polish, because even at the Good level the diamond constantly looks like it needs cleaning. Since polishing is graded, it is best to get a GIA (Gemological Institute of America) certificate in order to certify that you have the proper grading to ensure you have the best brilliance you can afford.