The psychology of it is the description of the way it influences us in our every day lives, and also is indicative of the way Jung attempted to describe it to people without turning them off, especially in the context of the early twentieth century, and his devotion to remaining a man of science despite his experiences. Jung's now famous ideas and his split from Freud started with a series of seemingly prescient dreams that predicted a flood of blood sweeping across Europe, immediately before the first world war.
He then went through a series of dreams that could be best (and not even close to adequately described) as a spiritual, psychedelic journey into the depths of his own unconscious workings. He had a natural talent for lucid dreaming that allowed him to go all the way down.
He straight up says he meets his soul (the anima), the old gods, and the ghosts of the dead within himself in these dreams, a long road down (down-going a la Nietzsche) to the point where he creates a new Godhead within himself, born out of this harrowing journey through the depths.
The collective unconscious is like the entire depth of the psyche that contains all this and exists throughout every living thing. He draws the parallel to the Atman himself explicitly.
Personally I'm a bit agnostic about the more spiritual elements, but fuck me if Jung wasn't the first guy to truly experience, know, and accept all aspects of himself.