Jesus was talking about the Holy Spirit. The Spirit had not yet been given, because Jesus had not yet been raised to glory. But later, those who believed in Jesus would receive the Spirit.
But when the Holy Spirit comes to you, you will receive power. You will be my witnesses—in Jerusalem, in all of Judea, in Samaria, and in every part of the world.”
It was shown them that their service was not for themselves but for you, when they told about the truths you have now heard. Those who preached the Good News to you told you those things with the help of the Holy Spirit who was sent from heaven—things into which angels desire to look.