All good stories have a beginning, middle, and an end. This is about the beginning. Not just the beginning, but the beginning of the beginning. This is about the Inciting Incident.
The Inciting Incident is that first thing, great or small, that sets in motion the sequence of events that constitute the plot. From the First Act to the Denouement, everything that happens in the story can be traced back here.
Despite the Inciting Incident's importance to the plot, it doesn't necessarily have to be a large event—it could be something as mundane as a phone call (in fact, in many non-heroic dramas it is a small thing of seeming unimportance.)
While the Inciting Incident may appear banal on the surface, it has some heavy lifting to do in a narrative sense. Firstly, it must propel the protagonist into the first phase of the plot: it sends the soon-to-be lovers into the Meet Cute in a Rom Com, it is the death of a loved one that inspires the hero to fight injustice, it is the French Toast being scraped into the trash by a emotionally cold mother. Secondly, it must place in the audience's mind the notion of what The Climax will be like: if it is the discovery of a shark-bitten corpse on a beach, expect a showdown at the end with said shark; if a farm boy discovers a message of intergalactic espionage, be sure he will follow his journey to the Galactic Overlord.
If the Inciting Incident is taking place in a Heroic Epic, it is a Call to Adventure. It's what the hero reacts to.
Note: The Inciting Incident need not be the first thing the audience sees, nor the first eventful thing to happen in the story. It can often be preceded by long bits of Exposition, glimpses into the Backstory, or scenes setting up the world of the story. It pertains to the protagonist solely, and his role in the overall plot.
No examples, please; this is an Omnipresent Trope