This question can be interpreted in at least two different ways, based on internal ambiguities. From one perspective, we can take it as a inquiry about those things we have knowledge of our own lack of knowledge about. I know that I do not know what day I will die, or what the afterlife might be like. I know that I do not know if there are aliens in outer space. I know that I do not know how to speak the Zulu language.

On the other hand, we can take the “that” as an equals sign, and the question becomes the paradoxical one, what is it that you both know and do not know (simultaneously). It is an easy temptation to dismiss questions like this out of hand, as meaningless nonsense, but as powerful thinkers from Ecclesiastes to Kierkegaard to Nils Bohr and beyond have noted, it is the most paradoxical concepts that tend to lead to the most powerful insights.

Unfortunately, in this case, no powerful insight arises for me –at least not immediately –out of contemplating this question, so for the moment, I will simply leave it with this respectfully inadequate answer.