Re-birth is one of the most important doctrines of our faith. When sin entered the world through Adam, the spiritual life of his soul died.
In John 3:3, when Jesus told the Pharisee Nicodemus that he must be born again to see the kingdom of God, Jesus was saying that his soul was in need of regeneration, i.e. being born again.
We know that after Jesus ascended to Heaven, He sent the Holy Spirit to regenerate repentant sinners and give them new birth, new life—eternal life. “You must be born again” means you must turn from yourself and turn to Jesus in faith, and trust in who He is and what He has done to pay for your sins. If you have not done that, read this:What If You Die Today?