The initiating event (sometimes called the inciting event) of a story is the event which sets the plot/central conflict in motion. It is not necessarily the point where the protagonist is drawn into the narrative, though it can be. This event can be shown in the story proper, or it may have happened before the story's opening scene. Regardless, it is the event which triggers the conflict of the narrative.
In Star Wars, the initiating event is Princess Leia giving the Death Star plans to R2D2 for safekeeping before she is captured by Darth Vader. This event starts the actual plot of the movie: the plans need to be delivered back to the Rebel Alliance before the Empire can retrieve them.
In To Kill a Mockingbird, the initiating event is when Atticus Finch is assigned the defense of Tom Robinson, a black man accused of raping a white woman. While a lot happens in the novel, the Robinson case provides the central conflict, pitting Atticus's fair-minded philosophy against a racist, narrow-minded small town willing to assume Robinson is guilty solely because of his blackness. Atticus's taking the case also causes problems for his two children, who are bullied by the other children and forced to confront their own ideas about prejudice as a result.