It refers to to the mystery of consciousness and the inner voice we experience.
Where does this voice come from?...
The McKenna quote is appropriately accompanied by the image of a thinking chimp, being manipulated by an alien entity- with what appears to be The Flower of Life over his third eye. This process of evolving consciousness is mediated by the psylocibin mushroom.
"Half the time you think you're thinking,
you're actually listening."