What does Galatians 1:22 mean?

The Judaizers were questioning Paul's legitimacy as an apostle, apparently suggesting he had merely been trained by the other apostles. If that were the case, Paul could not rightly call himself an apostle. An apostle, in New Testament terms, mostly referred to someone who had spent time in Jesus' company and who had been sent by Christ to do a specific work.

Paul has shared his story of coming to Christ. God had revealed His Son to Paul. The Lord had, indeed, sent Paul to preach to the Gentiles. After his conversion, Paul had spent three years off on his own followed by one brief stint in Jerusalem with Peter and Jesus' brother James. He had then gone to Syria and Cilicia, nowhere near Jerusalem and Judea.

That's why he says in this verse that he was an "unknown person" in that region. His point is that he learned all he knew about the gospel of Jesus while he was away from the other apostles. Those great truths were revealed to him by Christ.