The meaning here is fairly simple:
The illiterate man is simply ignorant.
As you don't know, you are ignorant.
The learned man is learnedly ignorant.
you are still ignorant, but in a different way.
the Sage is also ignorant... because there is nothing for him to know
This is the unusual one, at least for people that have been raised with traditional western culture. The ultimate "knowing" in all of Buddhism, Taoism, Hinduism, and Zen, is the state of enlightenment. The "Sage" is the master that has found enlightenment. In that state, the mind is ultimately silent. It is a state beyond thought. Knowledge, is a product of the mind. The Sage has gone beyond thought, and so "There is nothing for him to know". He is in "emptiness", the principle of Buddhism. There is nothing, or NO THING.... as "things" only exist as they are perceived by the human mind.
What do you think now that you have heard this explanation? I am curious.