Approach to the Innermost cave is what this segment of the journey is called.
Does he mean a real cave. I use them in many of my stories because that is where the hero or heroine learns things. Actually I don't think this is what it means at all. This is the time when the focus character steps into the territory of the villain or the one of his biggest fears. This can be a scary place and comes just when the hero or heroine is facing their goal. The fear of failure plays a part here and builds to bring suspense to the story.
But back to caves. Caves are places of mystery, sometimes beautiful and sometimes deadly. There are also creatures in caves and a chance for solitary thought, something the hero or heroine needs. This is a place where the goal can be thought of and perhaps even changed. A cave also represents the womb and when the hero or heroine emerges they are in a sense born again.