The disciple's great fear is that if Jesus goes away, they will know and experience Him less. Jesus explains to them that in fact, the opposite is true. The disciples will know more of Christ because He will send His Spirit and the Spirit of Christ will dwell with them and in them. And Christ assures them that despite the trials and tribulations they will experience, Christ will give them his joy and his peace to be in them.
Jesus is preparing his disciples for his departure. The world has largely rejected his revelation of the Father and his Word. As a result, the hatred of the world for Christ will lead them to crucify him. Before He is arrested, Jesus shares with his disciples that as they bear witness to him and bear his Name, the world will also hate them. Persecution is coming. The disciples will be hated for Christ's sake. Jesus shares this prophetic word with his disciples so that they will not fall away when the persecution comes. But rather remember Jesus has told them what is to come and have their faith in him strengthened, not weakened through persecution. And as we reflect over the course of church history, we realize this prophetic word from Jesus was true not just for the Apostles, but for the church at large.