A story is usually told through the mind of one or more character. It’s known as the story’s point of view (POV). The reader is only allowed to know what the POV character perceives and experiences.
Author intrusion is when the author invades the story, telling the reader something the protagonist doesn’t know. Often we see this written, “little did he know…”
Unless the story is told in first person an author intrusion “interrupts” the flow of the story by placing the author somewhere into the story.
EXAMPLES: Third person POV: “Sarah’s nerves were on edge, unsure of what she was seeing.” Author intrusion: “If we look at Sarah, we see she is hesitant about getting involved.” “By Tuesday, Jean would never want to see Frank again.”