Nothing will ever be the same again...
Aladdin at the Cave of Wonders
After having heeded the call to action (or by denying it becoming embroiled nonetheless), and having met with or received their first instance of supernatural aid, the hero comes to their first threshold. In the monomyth, as delineated by our good friend the comparative mythologist Prof. Joseph Campbell, the crossing of the first threshold is the point in the hero's story where they physically or mentally move past the known limits of his or her world and enter into places previously unknown to them. In this new place, the rules and limits of their old lives no longer apply.
Unknown worlds lie ahead
Upon reaching the threshold, the hero must undergo some sort of ordeal in order to pass into the world of adventure. The qualifying trial may be something as simple as entering a dark cave or woods, or as wild as being sucked up by a tornado. What is most important here is the contrast between what is familiar to the hero and what is unknown.
Campbell: "The usual person is more than content, he is even proud, to remain within the indicated bounds, and popular belief gives him every reason to fear so much as the first step into the unexplored. The adventure is always and everywhere a passage beyond the veil of the known into the unknown; the powers that watch at the boundary are dangerous; to deal with them is risky; yet for anyone with competence and courage the danger fades."
In some versions of the hero's journey, the threshold is protected by a guardian or "gatekeeper" who tests the hero’s competency before allowing entry into the new realm. The gatekeeper at the threshold to adventure can be seen as an inversion of supernatural aid. Whereas supernatural aid represents the natural help offered to the hero by nature or the protective universe, gatekeepers are usually terrifying creatures, monsters and shape-shifters that represent the amorphous world of the unknown.
As in the case of supernatural aid, there need not be a character physically manifesting the gatekeeper. At the entryway into the world of the unknown, it is really the uncertainty within themselves that the hero is fighting and must conquer to move forward. The 'gatekeeper' can very well just be their own worry and past fears telling the hero they are not prepared for the journey ahead. Defeating the gatekeeper is defeating all the thoughts that have kept the hero complacent in their ordinary world.
The new world the hero moves into, the regions of the unknown (desert, jungle, deep sea, alien land, etc.) are free fields for projections of unconscious content. It may be a literal dream world that the hero enters into after passing the threshold of sleep: the subconscious, the fluid place of fears and uncertainty. There is a sense of danger inherent in crossing the threshold from the known, but an offering of opportunity is conveyed as well. Having crossed the threshold, the hero is reborn into a new world, and may appear to those left behind as dead or lost.
The threshold is the 'line in the sand', the farthest reach at the end of town, the glowing gate in the pyramid. Having accepted the call to adventure, and receiving the blessing of the universe in the form of supernatural aid, the hero sallies forth, reborn anew into a world wholly unfamiliar, where they will be transformed and granted the power to return to their old world, a new person, a hero.