In other words, not much. All it means is that you acknowledge receiving it. But if you don't sign it, you could be thought to be insubordinate. In some states, you are allowed to place a rebuttal in your file if you do not agree, but I'm not sure if NY is one of them. Even if it is not, and you want to write a coherent, calm rebuttal, you can still do that and give it to your manager, then I would make an appointment to discuss.
I actually did this once when I got an appraisal from a new manager who had no idea what I did. When I rebutted several points with examples, he changed the review.