What does Matthew 28:18 mean?

Jesus is appearing to His remaining disciples, who number eleven now that Judas is gone. They have come together on an unnamed mountain in the region of Galilee in the northern part of Israel, away from Jerusalem (Matthew 28:16–17). Jesus tells them what He has often talked about in their presence before His death and resurrection: absolute authority has been given to Him by God the Father. In other words, the source of all power has given all His authority to His Son Jesus.

Here's what Jesus said in Matthew 11:27, long before His death and resurrection: "All things have been handed over to me by my Father, and no one knows the Son except the Father, and no one knows the Father except the Son and anyone to whom the Son chooses to reveal him."

Jesus, the Son, has chosen to reveal the Father to His eleven disciples standing before Him. He will now give them the authority—and the responsibility—to represent Him and His salvation to the world. This verse and the two to follow at the end of Matthew's gospel are often called together "The Great Commission."