What does Acts 26:16 mean?

This is part of Paul's testimony before the leaders of Caesarea Maritima, giving context for their benefit—specifically King Agrippa II. Paul is describing how he first met Jesus on the road to Damascus as recorded in Acts 9:1–19. We shouldn't expect the wording to be exact. Some of the words were not recorded in Acts 9. Some may be from what Ananias told Paul later. Others are representative.

God telling people to rise is a theme in the Bible. When Ezekiel fell to his face, overwhelmed by God's glory, God told him to stand so he could hear what God had to say (Ezekiel 1:28—2:1). Daniel, often overwhelmed by God's messengers, was told to stand (Daniel 8:18; 10:11). And Jesus had to tell Peter, James, and John to rise when they were overpowered by the voice of God on the Mount of Transfiguration (Matthew 17:6–7).

"Witness" is from the Greek root word martys. It refers to someone who provides testimony to what he or she has experienced. Often, it refers to the Christ-followers who told others they had seen Jesus after the resurrection. But martys is also the source of the English word martyr. A martyr bears witness to the good news of Jesus not only with words, but with their lives, forfeited at the hands of those who reject that message. When Jesus told Paul he would be a servant and a witness, He's saying Paul will live and die to spread the good news of Jesus to the Gentiles (Acts 26:17; Colossians 1:24).