What is the spiritual meaning of unfoldment?

I do not wish to convey the idea that spirits are all of one mind or talent, for they are as varied in organisation, talent or aspirations, as the flowers of the field, but they are all on the plane of spirituality, as one or a band or class.

As they become prepared to enter another sphere, they are ushered into the next higher with great rejoicing where they are received and accorded their position, preparatory to a still higher unfoldment.

―Mary Washington—Social Life of Heavenly Spheres