After the hero has crossed the first threshold, they enter what is known in the hero cycle as the Belly of the Whale. Its title is a reference to the story of Jonah being tossed to the whale in the Old Testament of the Bible. At this point in the story, the hero enters a zone of great danger.
Prof Joseph Campbell - "The temple interior, the belly of the whale, and the heavenly land beyond, above, and below the confines of the world, are one and the same... The devotee at the moment of entry into a temple undergoes a metamorphosis. Once inside he may be said to have died to time and returned to the World Womb, the World Navel, the Earthly Paradise. Allegorically, then, the passage into a temple and the hero-dive through the jaws of the whale are identical adventures, both denoting in picture language, the life-centering, life-renewing act."
What this means is that the crossing of the first threshold can really be seen as an act of self-annihilation. The hero goes inward, to be born again. This is symbolized in many religions by the entrance into a temple, where one is cleansed and reborn. The belly of the whale is the point in the story in which the hero reaches their 'lowest point', or rather their 'point of least self'. That which they had known in their world before the call to adventure and crossing of the first threshold must be transformed or discarded for them to re-emerge triumphant and ready for the rest of their journey.
The imagery of being "inside" something here is important. The hero appears to those outside the "whale" as dead or gone, but where they have really gone is inside themselves. Their old self has indeed died. "Inside" can mean a literal womb or a tomb, a cave or a pyramid, or more figuratively, the hero's mind. What this inside represents is a place of transformation. Death and birth are one in this stage. It is a return to the cosmic forge, where personality and character are redefined and re-conceived.
This stage of the hero's journey can be depicted many ways, but the psychological aspect is always the same: Whether it be Hansel and Gretel being taken in by the evil witch who hopes to eat them, or Indiana Jones entering the secret tomb to steal the legendary idol, the Belly of the Whale is a place where the hero must go to lose those parts of the themselves that were holding them back in fear or ignorance, to transfer to them new knowledge of self, powers of understanding that will carry their new form through the rest of their quest back to the world they left behind when they first fell the call to adventure.