What wakes your character up? What puts them into motion and pushes them to explore something new? This is the Call to Adventure!
The thing that calls your character from their ordinary world into the world of the story can come in a variety of ways. Like I said, it's a question. So, it might come in the form of someone asking a question. Think of Hagrid showing up to offer Harry a spot at Hogwarts. That someone often becomes a mentor to the MC--Hagrid, Obi Wan Kenobi, the White Rabbit, are all characters who serve as mentors and deliver the question of the Call to Action.
The call might be internal. A longing inside the hero, like an itch they are compelled to scratch. Dorothy is literally picked up by an act of God and put back down in the special world of her story, but the reason she isn't safe in the storm cellar where she should have been is because she was drawn to run away by an inner longing for something she couldn't find at home. Anyone who watches The Wizard of Oz feels Dorothy's longing when she sings Somewhere Over the Rainbow.
Let's dive in and explore what makes this moment in your story essential to getting things moving!
