Manifestation is a rather daunting word. But the manifestation of the Spirit is nothing spooky. It doesn't even necessarily mean supernatural things from God. The Spirit of course is God the Holy Spirit, the third person of the trinity. He manifests himself, not in spooky ways, but in the everyday way we use the gifts He gives to us.
Manifestation means clear evidence that something is there.
It means the Spirit showing us that He is living and working in a person. God gives you the clear evidence of the Spirit working in your life, not just so that you can grow in personal holiness, not just so that you can understand the bible better, or pray more often, not just so that you can feel closer to Jesus, but for the common good of your Christian brothers and sisters. It's that you are committed to the people of God, the church. That you are concerned to use the gifts you have to build the church. If you don't have the Spirit of God, then you won't care about his people, or about extending his kingdom by bringing others in.