Acts 22:9

And they that were with me saw indeed the light, and were afraid; but they heard not the voice of him that spake to me.

This is part of Paul's account of his conversion on the journey to Damascus, when suddenly a great light from heaven shone around him. He related this account to the crowd at Jerusalem as evidence of how he had come to serve Christ and to preach to the Gentiles. Those who were with him saw indeed the heavenly light and were afraid. They did not hear the voice of Him that spoke to him, while he alone received the spoken message that directed his work.

They did not understand the words spoken to Saul, while Saul alone understood the voice and recognized that Jesus was speaking to him. In that encounter Jesus revealed Himself to Saul, teaching him that He was the Son of God. His conversion and call to take the gospel to the Gentiles was connected with a vision in the temple, where God directed him to go far away unto them.