Meditation
A tool for quieting the intellect. Explicitly not the goal, and explicitly not tied to any one technique.
Campbell's own description, from his forum answers, is unglamorous and useful: meditation is a technique you use to quiet the constant jabber of brain-based thoughts and ego, all of which are about me in this reality — an intellect chattering away about nothing and everything.
He puts its role in one sentence: meditation is no more than a tool to help you control the intellect and thus enable you to find and integrate with the core of your being. The work itself is elsewhere — in the fear.
He is equally direct about technique. All a mantra does is push extraneous thoughts out of your mind, and imagining a soccer ball or following your breathing does exactly the same thing. There are, he says, dozens and dozens of doorways, all leading to the same place. The one you are least likely to walk through is whichever one your ego has decided is the right one.
Where this gets misread
That getting good at meditation is the achievement. It is scaffolding. Campbell's own stated timeline is worth having in advance: roughly three to nine months to become sufficiently proficient at meditation, and three to nineteen years to clear out the fear, beliefs and ego that limit your awareness.
Campbell's own words, from the compiled archive of his forum answers.