"The young man then quoted with approval what Socrates had said on learning that the Delphic oracle had proclaimed him the wisest man in Greece. "If I am the wisest man," said Socrates, "it is because I alone know that I know nothing."

This is a common misquote with similar frequency to perhaps "Play it again, Sam." In his Apology (not meaning apology, from apo-logia, speaking for oneself, i.e. defense) Socrates says:

"... ἔοικα γοῦν τούτου γε σμικρῷ τινι αὐτῷ τούτῳ σοφώτερος εἶναι, ὅτι ἃ μὴ οἶδα οὐδὲ οἴομαι εἰδέναι."

"Although I do not suppose that either of us knows anything really beautiful and good, I am better off than he is – for he knows nothing, and thinks he knows. I neither know nor think I know" (https://en.wikipedia.org/wiki/I_know_that_I_know_nothing?utm...).

The context is that Socrates was puzzled by the Delphic Oracle's declaration that he is the wisest man. The man mentioned is an unnamed politician who was known to be wise.