Polishing is definitely a case-by-case basis, however it typically would aim to assuring the game speaks for itself.
Generally speaking, you will learn most of what you need to know through playtesting. During a playtest, you should be able to sit silently, not explain anything, and observe the player go through your title as anticipated. If they don’t, then note where and how they got stuck, then fix it. Repeat this process a few times and you will officially have “polish”!