As the name suggests, final polish is the last step of the process, after which the content is officially done and ready for release. This step will often be offset in time and can take place long after all art and gameplay aspects have been implemented, with artists and designers working on something else in the meantime.
Why can't we simply finish everything in one go? You can certainly try, but with the passing of time you'll improve your skills and gain new perspective and insight. You'll also be able to compare your creation with the rest of the game and reflect on it as a part of something bigger. You are also likely to get feedback from multiple rounds of external and internal playtesting of the game.
Extensive iterations ...
