What does “in the world of possessions” mean?

Here, “in the world of possessions” means the world where everything and every action is made to possess something, whether it is the possession of land, property, money, or any other thing. The poet suggests that losing a ball, which is a very small thing, would make the boy understand what it is like to lose something that one possessed. This would make the boy realise that this is a world of possessions and where one can possess more things by buying them, one cannot buy what has been lost.