An inciting incident is the first occurrence in the beginning of a short story, novel, novella that a character faces, it is the action part, it can be a crisis, a desire, or a yearning, an opposition that must be resolved through change, by the ending.
Some refer to this as the hook, an inciting incident doesn’t have to be huge, like a murder or a bank robbery, but it is the question that sets up the story, and creates an event that throws the characters everyday life out of balance and triggers them into taking action.
The character does not always have to be willing, either, there can be internal conflict in the main characters head or external between one character or another or some outside force that creates conflict, for your main character.
Campbell calls it a hero’s journey.
In his book “The Hero with a Thousand Faces” he summarized the monomyth:
“A hero ventures forth from the world of common day into a region of supernatural wonder: fabulous forces are there encountered and a decisive victory is won: the hero comes back from this mysterious adventure with the power to bestow boons on his fellow man.
In laying out the monomyth, Campbell describes a number of stages or steps along this journey.
The hero starts in the ordinary world, and receives a call to enter an unusual world of strange powers and events (a call to adventure). If the hero accepts the call to enter this strange world, the hero must face tasks and trials (a road of trials), and may have to face these trials alone, or may have assistance. At its most intense, the hero must survive a severe challenge, often with help earned along the journey. If the hero survives, the hero may achieve a great gift (the goal or "boon"), which often results in important self-knowledge. The hero must then decide whether to return with this boon (the return to the ordinary world), often facing challenges on the return journey. If the hero is successful in returning, the boon or gift may be used to improve the world (the application of the boon).Wikipedia article link: The Hero with a Thousand Faces
Joseph Campbell describes the inciting incident the best here this part of the article:
The Hero with a Thousand Faces
1. The Call to Adventure
The adventure begins with the hero receiving a call to action, such as a threat to the peace of the community, or the hero simply falls into or blunders into it. The call is often announced to the hero by another character who acts as a "herald". The herald, often represented as dark or terrifying and judged evil by the world, may call the character to adventure simply by the crisis of his appearance.
It’s what upsets the balance of your protagonist life or the forces of your story. The opening of a story has to have a compelling reason for the reader to care.