What does Galatians 1:16 mean?

Paul is telling his personal story of coming to Christ in order to convince the Galatian believers that he did not invent what he taught them. He received it from Jesus directly. After a zealous career as a Pharisee, God intervened in Paul's life. Paul understood now that God had set him apart to fill this role of being an apostle before he was even born. God was the one who called him. Paul knew he did not earn or deserve this job; it was grace.

Specifically, Paul now describes what God called him to do when He revealed to Paul His Son, Jesus (Acts 9:1–22). Paul's mission was to preach about Jesus to the Gentiles. That's what he was born to do. It's what God commissioned him to do.

Paul now comes to an important point in his argument that the gospel he taught them did not come from other human beings. When, by God's grace, Paul believed in Jesus and understood his calling, he did not immediately go find some person to start training him in the things of Christ. As the following verses will show, he went off by himself, instead. The significance of this is that the gospel Paul now preaches isn't something he is simply regurgitating from his teachers. It came to him directly from the Lord.