What is the initiating event in a story?

The initiating event is the first stimuli in a story that triggers the beginning of the story’s primary conflict.

For example:
In “Harry Potter and the Sorcerer’s Stone”, the initiating event could be considered to be when the trio first uncover a three-headed dog guarding an unknown object, which triggers their curiosity as to what it might be and then leads to the rest of the story.