That which is born of the flesh is flesh. That which is born of the Spirit is spirit.
Jews prided themselves that they were born Jews, and thought this guaranteed their entrance into the kingdom of God. Jesus was not concerned with physical birth or an earthly kingdom. He was talking about the work of God’s Spirit that gives repentant sinners new life and so enables them to enter God’s kingdom. Like the wind, the work of the Spirit is mysterious. It cannot be seen, though its results certainly can (John 3:6-8). Those who have personally experienced God know this new life. Those who think only in terms of earthly things will never know it (John 3:9-12).