From what I read, it says that the electric field, or the magnetic field on a electromagnet, is proportional to the amount of electricity running through an object.
Obviously, the material within the object changes the amount of electricity an object holds in time, so that the electric field on a plastic stick with electricity running through it would be minute. The conductivity of the plastic reduces the field, therefore making it difficult for a plastic stick charged with electricity to pick up metal objects.
I am twelve years old. This is the way I interpret it.