Extraneous details are the details that are not connected particularly with the passage. These details are not necessary to be added to the story.
Explanation:
My mum makes the most delicious cookies. She even makes them in different shapes every Saturday morning. My sister is not very hungry. I love the drizzling chocolate all over the cookies mum makes.
In this passage, the sentence <em>My sister is not very hungry </em>is irrelevant to the topic of the paragraph.
