You don’t know what you don’t know. There is humility and maybe even freedom in this truth.
The antithesis, “you only know what you know”, but some people only operate in the space of everything they know with arrogance and conceit as if there is nothing more to know beyond what they know. It takes vulnerability, exposure to be okay with not knowing something, learning or taking on a beginner’s mindset. Which is not very comfortable. Operating in the discomfort, awkwardness of the unknown, curiosity, exploration.
But the real knowledge is actually knowing that you don’t know what you don’t know. It’s the first prompt of curiosity to seeking out those things you don’t know.