It refers to the bringing down of the Supramental consciousness, and the resulting transformation of the entire being.
From this base it is progressively awakening through the life of the plant, the beginnings of mind in the animal, the full emergence of mind in humanity, and is now stirring to awaken fully through the emergence of a greater consciousness than mind, the Supermind, in which the fullness of the undivided consciousness and infinite delight of the One will be manifest in individualities embodied here on earth.
The purpose of the journey is to settle Trevize's doubt with his decision at the end of Foundation's Edge to embrace the all-encompassing supermind of Galaxia.