In this passage it is just after Jesus has rose form the dead (he is alive and lives in heaven and our hearts) he tells them to listen to him and stay in Jerusalem (God speaks to use and tells us were he wants us to go) he tells them that the Holy Spirit is coming. The holy spirit was a promise from God sent after the death and resurrection of Jesus. The apostles are still thinking Jesus is going to lead a revolution and free Israel from the Romans (they still didn't fully understand Jesus which mean we will never understand all of Jesus/God). Jesus said that God not him is the only one who can reveal dates and times to a person (the prophets and Jesus only knew what God wanted them to know). When the Holy Spirit arrives they will be his witnesses to all the world (he may send you out and he will take care of you).

Acts 1:1-8