"What does it mean to believe in Jesus"
Believing in Jesus means that you believe that Jesus is God in human form (John 1:1, 14). You believe that Jesus died on the cross to pay the penalty for your sins (1 Corinthians 15:3; 2 Corinthians 5:21), for which you deserve eternal separation from God (Romans 6:23). You believe that the sacrifice of Jesus, God incarnate, is the only adequate payment for your sins (1 John 2:2; John 14:6; Acts 4:12. So when you say that you believe in Him, you believe He is who He say's He is, what He say's He's done and what He say's He's going to do.