The inciting incident is the moment the story proper begins — the disruption that pulls the protagonist out of their ordinary world and into the central conflict. It usually arrives early, after just enough setup for the reader to feel what's being disrupted.
A strong inciting incident is specific and irreversible: once it happens, the character can't simply go back to how things were. It's the first hinge of three-act structure.
Example
The letter that reveals the inheritance; the phone call about the body; the stranger who knows the protagonist's real name.