Something that's polished has been perfected — either by being rubbed until it's shiny or practiced until it appears skilled and elegant.
A polished diamond is bright, with a smooth, shining surface that reflects light. For an object to be polished, someone has to polish it, rubbing and buffing until it shines. You can also have a figuratively polished version of your essay that's been spruced up through careful editing. If a person is polished, they're graceful or accomplished: "Her polished style of speaking was almost intimidating!"