Human Wisdom

At 23a in the Apology, Socrates says:

What is probable, gentlemen, is that in fact the god is wise and that his oracular response meant that human wisdom is worth little or nothing, and that when he says this man, Socrates, he is using my name as an example, as if he said: "This man among you, mortals, is wisest who, like Socrates, understands that his wisdom is worthless."

• What leads Socrates to say this?
• What does Socrates mean when he says "human wisdom is worth little or nothing"?