Original: The Ascension of Jesus is the Christian doctrine from the New Testament of the moment when the resurrected Jesus was taken up to heaven in his resurrection body, in the presence of eleven of his Apostles.
Simplified: According to the New Testament, the Ascension of Jesus happened at the end of Jesus' life on earth. The Acts of the Apostles says that after Jesus told his apostles to teach the gospel to everyone, he went up to heaven to be with God.