"I know that I know nothing" is a saying derived from Plato's account of the Greek philosopher Socrates: "For I was conscious that I knew practically nothing..." (Plato, Apology 22d, translated by Harold North Fowler, 1966)
Socrates did the 'I am humble' but in reality I am hiding my arrogance position well. He used the declaration as a challenge and often proved his own claim. An extremely overrated philosopher who himself produced the rationale for totalitarianism.
I know that this does not mean that I know nothing at all, but there have been times when I have entered a subject and honestly knew nothing!
When I was an undergraduate student with the Open University, I did their science foundation course and there was a section about quantum mechanics, and I literally did know nothing. The nearest I had got to knowing any of the words involved was from the Hawkwind (rock band) track 'Quark, Strangeness and Charm'!
This is commonly attributed to Socrates, but as he was equally accused of arrogance and conceit, and only pretending to be humble, we can view this as an example of both. He is/was better than other thinkers because he accepted his lack of (certainly ideological/social/) standpoints. Nevertheless, his method of debate was the negative dialectic involving the destruction of his opponent's viewpoint and the imposition of his own.
This is not a rhetorical device and not an understatement, but this is the idea at the core of this philosophy. We do not have dogmatic truths at hand, but we can only approach truth step by step by the dialectical method. It's just the foundation of rationalism. Therefore, Plato thought also about the handling of likely "myths", because everything we "know" is only more or less likely. Basic formal and metaphysical knowledge excluded (such as 2+2=4, or: I know that I know nothing). This is at least Plato's view on the matter. Modern thinkers do more differentiation, of course.
By the way, since the topic was mentioned here: Plato developed indeed the idea of an ideal state, and if realized, this ideal state would practically end up on a commuism-like totalitarianism. But Plato also changed his mind later and started to develop a "mixed" constitution, i.e. what we today call a Republic (he did this in the dialogue The Laws, not in the dialogue The Republic. Confusing, I know.)